You can substitute whole milk for heavy cream in recipes that call for a small amount of cream to thin the frosting. Recipes that rely on heavy cream as a main ingredient, such as ganache, don't adapt well to whole milk. Also, consider the event you are serving the cake at, as well. You'll probably feel more comfortable experimenting with a casual family dessert than a dessert for a holiday gathering or other important event.
Use whole milk to thin a simple butter and confectioner's sugar icing instead of heavy cream. The purpose of the liquid is to thin the icing to a spreadable consistency, so you'll probably need slightly less milk than heavy cream. Soften the butter first and sift the confectioner's sugar to remove any lumps. Add 2 tbsp. whole milk at a time, beating after each addition until you have the desired consistency. If you get it to thin, add more confectioner's sugar. Keep in mind, though, that the icing will thicken slightly as the butter hardens.
Use whole milk instead of heavy cream to thin powdered sugar glazes, such as those used for cinnamon rolls or doughnuts. Beat 1 cup powdered sugar with 1/2 cup butter until smooth. Add whole milk until the glaze is the consistency of pancake batter. Drizzle the glaze on freshly baked pastries and allow it to set for a sweet taste.
Whipping Cream Frosting
If the recipe calls for an icing of whipped cream, such as a chocolate roulade, try whole evaporated milk instead. Chill 1 cup evaporated milk in the freezer for 30 minutes, as well as the beaters and bowl. Whip the evaporated milk for two to three minutes until it is stiff. Add 1/4 cup powdered sugar and 1 tsp. vanilla extract.
If the recipe calls for ganache and you don't have heavy cream, your best bet is probably to select another cake or substitute a different type of icing. For example, chocolate buttercream is delicious on most chocolate cakes instead of ganache and requires only chocolate, butter, sugar and eggs. A seven-minute frosting is made by warming egg whites and beating them with sugar until they are frothy and light, but still moist. This frosting is shiny, sweet and delicious, and makes a good alternative to whipped cream for chocolate or coconut cakes.