1 cup (227 g) unsalted butter, softened
6 cups icing sugar (confectioners')
120 ml whipping cream
2 tablespoons pure vanilla extract
1 tablespoon water
pinch of salt
Beat the butter and icing sugar in an electric mixer on low with the paddle attachment for about 2 minutes. Add the vanilla, water, whipping cream, and salt, and whip on high speed until fluffy and smooth--about 4 minutes. If consistency is too thick, add more water 1 teaspoon at a time, then whip again for 30 seconds or so.
Makes enough to fill and frost a 3-layer 8" cake.