Ingredients:
-4 sticks unsalted butter, at room temperature
-about 6 1/2 cups confectioners' sugar, sifted
-1/4 cup plus 2 tablespoons Irish liqueur cream
Instructions:
In the clean bowl of a stand mixer, beat the butter until light and fluffy. Gradually add the confectioners' sugar then the cream. Beat the mixture on high speed until very light, fluffy, and smooth, about 3 minutes.
Assembly:
Place the cake layer, flat side up, on a cake stand. Place pieces of parchment paper underneath the cake to protect the cake stand from buttercream. Remove some of the buttercream to a pastry bag fitted with an open tip, or to a large zip-top bag with a medium-sized hole snipped in the corner, and pipe a ring of buttercream around the edge of the cake. Frost the inside of the ring on top of the cake generously, as this will become the filling. Top with the other cake, flat side up. With an offset spatula, spread a very thin layer of frosting all over the cake. This layer is a crumb coat and should act like spackle. Return the cake to the freezer until firm, about 20 minutes. Spread the remaining frosting all over the cake. Let come to room temperature before serving.