For the dough, stir the butter until foamy. Then stir in powdered sugar, cocoa powder, salt and vanilla extract thoroughly. Work in the flour so that a soft, but not sticky dough is formed - add a little more flour if necessary. Knead lightly and refrigerate for an hour.
Roll out the dough 2 mm thick between two layers of baking paper, then cut out circles three centimeters in diameter. Place the biscuits on a prepared baking sheet.
Bake in a preheated oven at 175 ° C for about ten minutes, then leave to cool.
Mix the ingredients for the filling with the mixer until creamy until the mixture is white and fluffy. Spread a blob of it on a cookie and cover with another biscuit.