HOW TO FILL AND FROST A LAYER CAKE
Getting Ready
   Cakes should be completely cool before being trimmed, cut, filled, and/or frosted. Warm cake layers melt frostings and cause layers to slip and slide. (See Making a Layer Cake for illustrated steps.)
    To Cut a Single Tall Cake Horizontally into Thinner Layers
   If the cake is just-baked and very tender, place it in the freezer for 20 minutes. Set the cake on a cardboard round, a piece of aluminum foil, or a decorating turntable or lazy Susan. If the cake is not level, trim it now. Cut a notch down one side of the cake so that you can line the layers up properly when you assemble the cake.
   To divide a layer, place one hand flat on top of it. Hold the blade of a long serrated knife against the side of the cake where you wish to cut it. Turn the cake counterclockwise (clockwise if you are left-handed) while you saw a shallow groove all around the cake, always at an equal distance from the top of the cake, so the layer will be even. Do not try to cut all the way through the cake the first time around. After you have cut the groove, continue to rotate the cake while cutting deeper and deeper into the groove, until the layer is free. Alternatively, once the groove is cut, wrap a length of strong thread or dental floss around the groove, cross the ends in front, and pull them until the layer is cut through. Slide a piece of cardboard or a cookie sheet under the new layer and lift and set it aside. Repeat if necessary.
   To Cope with Dome-Shaped Layers
   Butter cakes may be slightly domed on top. In assembling a layer cake, the bottom and middle layers are usually placed dome side down and the top is placed dome side up. If the domes are very pronounced, trim them level and place them all upside down, since it is easier to spread frosting on an uncut surface.
    Creating a Base for the Cake
   A cardboard cake round or springform pan bottom the same diameter as the cake to be filled and/or frosted is a convenience that allows you to move the cake safely after you have completed it, simply by sliding a metal spatula underneath it and lifting. Anchor the cake to the base with a dab of frosting, to keep it from sliding as you work. Otherwise, assemble and frost the cake directly on the serving platter. Slide wide strips of wax or parchment paper under the edges of the cake on all four sides to keep the platter clean. Remove the strips after the cake is frosted.
    Using the Right Amount of Frosting and Filling
   The amount of frosting and filling used for a particular cake depends on the style of cake and the richness of the filling. Fluffy frostings and whipped cream are spread thicker than denser, richer buttercreams. The buttercream between multiple thin layers in European cakes should be spread less than 1/8 inch thick (only 1/4 to 1/3 cup of buttercream between 8-inch layers, 1/3 to 1/2 cup between 9-inch layers). Frostings and fillings between thicker American butter cake layers can be spread up to 1/4 inch thick (up to 1 cup between layers). In other words, if the layers are thin, spread the filling thin; if the layers are thicker, spread the filling thicker. (See individual recipes or the chart, for general guidelines.) Each time you set a layer in place, make sure it is centered on the layer beneath it, sliding it as necessary.
    Moistening a Cake with Soaking Syrup
   European sponge layers and génoise are usually brushed, sprinkled, or sprayed with a flavored soaking syrup,  to moisten them, add extra flavor, and create a liaison between the cake and the buttercream. Allow up to 1/4 cup syrup for each 8-inch layer or up to 1/3 cup for each 9-inch cake layer (do not count any dacquoise or meringue layers, which are not moistened). The bottom cake layer is liberally moistened only on top with the entire amount of syrup allotted for the layer; other layers are moistened on the top and bottom, with half the syrup allotted to each side. Prepare the syrup according to your taste and the sweetness of the liquor. Sweet liquors may be mixed half and half with simple syrup (some of the sweetest liquors can even be used without syrup). For high-proof or stronger liquors, try one part liquor to three parts syrup and adjust from there. If the cake is very dry, use more syrup; if a génoise came out especially moist and tender, use less. Also consider how thick the layers are—use less syrup than is called for on 1/4-inch-thick layers and up to the full amount on 1/2-inch-thick layers.
   Adjusting the Consistency of Frostings and Buttercreams
   The consistency of the frosting is critical to achieving the effect you want, in both filling and frosting the cake. Frosting that is too thick or stiff tears the cake and pulls up crumbs. Frosting that is too thin oozes out between cake layers, collapses into a puddle when piped, and may even slide off the cake. Modify the consistency of frostings according to need. Frostings rich in butter or chocolate can be softened or stiffened by placing the bowl in a pan of either warm or ice water and stirring to the desired consistency. Treat powdered sugar icings the same way, or beat in extra liquid to soften, powdered sugar to stiffen.
    Whipped cream frosting works a little differently; do not try to soften or stiffen it once it is made, and do not warm the spatula as you work, or you will break down the cream. If you are filling or frosting with whipped cream (or whipped ganache), underwhip the cream slightly. Spreading and smoothing the cream as you frost the cake will stiffen it adequately. If the cream is stiff before you begin, your finished frosting will look and taste overbeaten and grainy.
    Dealing with Crumbs in the Frosting
   Brush loose crumbs from the cake layers as necessary while you work. If a stiff frosting is tearing the cake and causing crumbs, adjust the consistency. Keep crumbs from “contaminating” the frosting in the bowl by scraping the spatula against another container each time before dipping it back into the frosting. Or divide the frosting between two bowls and work out of only one; save the other for the final frosting.
    “Crumb-Coating” and Finishing the Cake
   To keep crumbs from marring the finished frosting or glaze, seal the cake by first spreading a very thin coating of frosting, the “crumb coat,” all over to smooth the surface and secure the crumbs. For the crumb coat, it is fine to use frosting that is already contaminated with crumbs. Some cakes are sealed with hot strained jam or preserves. You can refrigerate the cake for a few minutes to set the crumb coat before applying the final frosting. For chocolate butter glazes and ganaches, cool the glaze to the consistency of frosting for the crumb coat, then gently warm the remaining glaze to the correct temperature and fluidity for glazing.
    With a clean, crumb-free spatula, spread the cake with a final attractive coat of frosting, smooth or in swirls as desired. You can texture the sides,  or coat them with chopped nuts, or use the back of a spoon to make peaks and valleys in the frosting—see decorating techniques.
    Each pastry chef has different tools that he or she swears by. A stainless-steel spatula with an 8-inch blade is an excellent all-purpose tool for spreading frostings on cakes that are 8 or 9 inches in diameter. The spatula will enable you to spread a frosting smooth, in swirls, or with raised spikes; it also facilitates the spreading of poured chocolate glazes. A cake comb or serrated knife enables you to texture the sides and/or top of a cake. Perhaps not surprisingly, a perfectly smooth coat of frosting is the hardest texture to master. If you are determined to learn this technique, you will find it useful to acquire a decorating turntable or a lazy Susan to enable you to rotate the cake, holding the spatula steady as you smooth the frosting.
    Any frosting or icing that is smooth and holds a shape can be piped with a pastry bag and a decorative pastry tip. Practice piping on the back of a cake pan.
    Powdered-sugar icings and buttercreams should be stirred briskly with a rubber spatula before use and from time to time to eliminate air bubbles and keep the icing smooth.