Melt butter in microwave and place 1/2 cup sugar in a brown lunch bag or bowl.
Brush each paczki while warm with the melted butter and place in bag to coat with sugar.
(I sometimes use cinnamon sugar mixture, instead of just sugar.)
Let them cool, then fill using a pastry bag with a long tip, pushing into the side. Squeeze custard filling in gently ( the paczki will grow a bit when filled with the custard.)
You may also fill you Baked Paczki with your favorite jelly/jam.