Icing serves many purposes in pastry -- filling cakes, topping off cupcakes and cakes, and creating edible decorations for pastries, to name a few. There are many forms of icing, but most recipes require milk as the liquid that brings all of the ingredients together. While you can certainly substitute milk with heavy cream, doing so means changing how your frosting is used and made.
Heavy cream, or heavy whipping cream, is thick and rich. While it's typically used for making whipped cream, it can also be used in its liquid form in recipes. Heavy cream has a fat content ranging from 36 to 40 percent, depending on the variety. A traditional icing recipe calls for butter, shortening, milk and confectioner's sugar to create a thin to stiff consistency frosting. Icing made using the traditional recipe can be used as a frosting, filling or decoration and can be tinted in a variety of colors.
Heavy cream can be substituted for milk in icing recipes. You can use equal portions heavy cream as a substitution for milk. Your icing may take on a thicker, creamier consistency than it would when using milk. Add your heavy cream at the same time you would add in the milk. Instead of whipping the icing at a medium-to-high speed, however, mix at a slow-to-medium speed. This keeps the whipping cream from expanding and adding too much air to your buttercream. Too much air in a buttercream leaves small air pockets, which result in holes and a non-smooth texture when you apply it to a baked product.
An icing that contains heavy cream won't be as stiff as an icing made with milk because of the additional milk fat. You can use icing made from heavy cream to fill, frost and cover cakes and pastries, but you cannot use it for decorations that need to hold their shape, such as flowers, swags or piped designs. Icing made with heavy cream can be tinted similar to icing made from milk.
Icing made from heavy cream should be refrigerated to avoid spoilage. Once the icing has been used on a pastry, refrigerate and consume the pastry within a couple of days. Unused icing made from heavy cream can be stored in an airtight container for up to two days, but it doesn't freeze well like icing made from milk. When frozen, it might separate or curdle when brought back to room temperature.