Use the paddle attachment on your mixer (this is your secret weapon for creamy buttercream). Cream the butter well for several minutes on medium speed. (If you don't have a paddle attachment, just use the whisk attachment, or a handheld mixer.)
Add in three cups of confectioner’s sugar and beat on low until incorporated and then medium until well mixed.
Add one tablespoon of milk and beat until incorporated, then add the last three cups of confectioner’s sugar and beat on low, then on medium until well mixed.
Add another tablespoon of milk and the vanilla paste or extract, then beat on medium until well mixed.
Now, check the consistency. If it’s too thick, add another tablespoon of milk. If it's too thin, add another cup of confectioner's sugar. Once you reach the consistency desired, beat on medium speed for about four to five minutes. The icing will lighten a bit in color.
This buttercream makes enough to cover a two layer, 8" round cake or about 24 cupcakes.Since this buttercream is made with all butter and no shortening, it will have a slight yellow or cream tone to it. If you want it to be very white in color, you can add a very tiny speck of blue or violet food coloring to cancel out the yellow, or just add white gel food coloring.You could also whip your icing for several more minutes once it's mixed. Whipping it seems to lighten the color a bit.You could also add clear vanilla extract instead of regular vanilla extract and that will lighten the color although the vanilla taste will not be as good.Now you’re ready to ice your cakes, cupcakes, or even just eat it with a spoon straight out of the bowl!