Cream butter with 4 cups of the confectioner's sugar. Beat in vanilla extract, 2 tbsp milk and remaining 2 cups confectioner's sugar into the mixture. Gradually add another Tbsp of milk into mixture if necessary and beat until smooth and desired spreading consistency. (For chocolate icing, add ⅔ cup sifted cocoa powder.)
Notes
Makes enough to ice and fill a 3-4 layer cake. I used it to frost one 3-layer cake and 24 mini cupcakes generously and had a little bit remaining.
Recipe by Hidden Ponies at https://hiddenponies.com/2013/05/saturday-sweets-birthday-cake-with-vanilla-frosting/