In general, all ingredients should be at room temperature (68┬░ to 70┬░F), or at about 65┬░F in a very warm kitchen or hot climate. This is especially important with butter cake recipes, where the butter, liquid, and eggs are meant to form an emulsion during the mixing steps. Emulsions can break or curdle (like a sauce) if some ingredients are colder than the rest. When this happens, the batter loses its ability to trap air, and the cake will be heavy.
Butter should be cool and firm but malleable when squeezed, not soft and squishy. Butter that is too soft or melted will not trap air and will collapse with prolonged beating. Your fingers are the best guide. Butter of the proper consistency may range in temperature from 60┬░ to 70┬░F.
Cold eggs cause problems in sponge and foam recipes, because they do not increase in volume as much as room-temperature or warm eggs when beaten. Cold eggs also spell trouble for chocolate recipes, as they are too cold to blend smoothly into melted chocolate or batters containing large quantities of melted chocolate.
Spur-of-the-moment bakers can transform cold dairy products into room-temperature ingredients quickly with judicious use of the microwave, a few seconds at a time, on low power. Break eggs and separate or beat them lightly before microwaving, and stir before checking the temperature each time. Liquids at 68┬░ to 70┬░F feel quite cool, not warm, to the touch, and butter is malleable but firm. If you are not sure of what this temperature feels like, an inexpensive instant-read thermometer can be used to verify your judgment. Absent a microwave, place whole eggs or a bowl of broken eggs in a bowl of warm water to bring them to room temperature. Do the same for liquids.
