Double the recipe to fill a 17 1/2 x 11 1/2-inch jelly-roll sheet generously. If you whip it until it is too stiff, warm the spatula with hot water to help in spreading. Start this at least several hours in advance.
Bring to a boil in a medium saucepan:
1 cup heavy cream
2 teaspoons instant coffee powder
Remove from the heat and whisk in:
4 ounces milk chocolate, finely chopped
Cover and let stand for 10 minutes. With a rubber spatula, stir until perfectly smooth, scraping the bottom of the pan to be sure all of the chocolate is melted. Cover and chill for at least several hours. (The ganache can be refrigerated for up to 5 days or frozen for up to 6 months.) To use, beat on low to medium speed just until the ganache is thickened and begins to hold a shape; do not overbeat. Use immediately.
