– 1/4 cup milk – 2 tablespoons cream cheese instant pudding mix, reserved from making the whoopie pie batter – 1 cup unsalted butter, room temperature (2 sticks) – 1/4 teaspoon salt – 1 teaspoon vanilla extract – 1 (7 ounce) container marshmallow cream (fluff) – 3 cups confectioner's sugar