On the Fly Chocolate Buttercream Icing

Today, I made chocolate cupcakes for the church picnic, but I didn't realize that I had no powdered sugar until I was whipping up the butter for the icing.  I should have probably thought of it; I don't usually keep powdered sugar on hand, as I almost never use it.  However, I winged the recipe, and it worked out great.  The kids were delighted.  So here it is:
6 T room-temperature butter
3-5 T milk or buttermilk (I actually had buttermilk because of another recipe, so I used that.)
heaping 1/4 c sugar
1/2 c cocoa powder

Mix well and enjoy.  It makes enough to frost 2 dozen cupcakes.