For the dough, beat the egg with the sugar until frothy. Mix yogurt with milk and vanilla sugar and add to the egg and sugar mixture. Then stir in liquid, cooled butter. Mix the flour with the baking powder and cocoa, then sieve and gradually stir in until a thickened mass is formed.
Place 1 tablespoon of dough each on baking sheets lined with baking paper and smooth out with a dampened spoon. The biscuits should have a diameter of about 3.5 cm. Important: leave enough space between the biscuits.
Bake in the preheated oven at 150 degrees on the middle rack for about 15 minutes. Then let cool down completely on a wire rack.
For the frosting, mix the butter, cream cheese and powdered sugar with the mixer until the mixture is creamy. Spread the frosting on each whoopie pie half and put a second half on top.