Whisk butter, brown sugar and oil until combined. Stir in egg and vanilla sugar. In a separate bowl, combine flour, cocoa, salt and baking powder and add to the mixture. Whisk at low speed. Add the milk and stir until you have a smooth batter, a bit thick.
Place small dollops of batter (or use a piping bag) onto an oven paper covered baking plate. Bake your whoppies for about 7-10 minutes. The tops should spring back when you lightly press the whoppies. Let the whoppies cool on a wire rack.
Meanwhile make the chocolate filling by combine everything together. Then pipe chocolate filling onto the flat side of the whoppie and press another whoppie on top.
The assembled cookies can be stored, covered, in the refrigerator for several days, if they last that long...
Comments:
You need to give measurement in grammes for flour and sugar as ml is used for measuring liquids.