Wilton.com has a good recipe for buttercream cake icing. Other good recipes for buttercream cake icing come from FoodNetwork.com and TheKitchenMagpie.com.
To make Wilton's recipe for buttercream icing, place 1/2 cup vegetable shortening and 1/2 cup softened butter or margarine in a large bowl. Beat with an electric mixer until fluffy and combined. Beat in 1 teaspoon clear vanilla extract. With the electric mixer running, slowly add 4 cups confectioner's sugar. Scrape the sides of the bowl frequently to incorporate all the sugar. Once all the sugar has been added, the icing should appear dry. Gradually add 2 tablespoons milk, and beat until creamy.
Food Network's recipe for quick vanilla buttercream icing omits the shortening. For this recipe, whisk together 3 cups confectioner's sugar and 1 cup butter. Mix until blended, and then add 1 teaspoon vanilla extract and 1 to 2 teaspoons whipping cream. Continue mixing until the icing becomes light and fluffy.
To make icing with a thinner consistency, add more milk or a few tablespoons light corn syrup. For icing with a stiff consistency, add more sugar, or omit the butter, and substitute shortening. You can also create buttercream icing in different flavors. For example, The Kitchen Magpie's recipe for chocolate buttercream icing uses cocoa, semi-sweet chocolate and coffee to add chocolate flavor.