Eleni, you mentioned this in my post about
DD's birthday. Here is the recipe I use. It's on the back of the powdered sugar box. I think Christy mentioned this before as well. It's always smooth - never grainy.
1 box confectioner's sugar
1/4 cup milk
1 tsp vanilla extract
1 stick salted butter (softened)
Throw it all in a mixer and mix for about 2 mins.