Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Line two baking trays with baking paper. For the dough, whisk butter, sugar and 1 pinch of salt with the whisks of the mixer for about 5 minutes until creamy.
Stir in the eggs one by one. Mix the red fruit jelly powder, flour and baking powder. Fold into the butter mixture with a dough scraper. Pour into a piping bag with a large perforated spout. Spray approx. 28 dough spots (approx. 6 cm Ø each) onto each baking tray.
Bake in the oven for about 15 minutes.
Remove the whoopies and let them cool on the tray for about 10 minutes. Then take them off the paper and let them cool down completely.
For the filling, whisk the mascarpone, icing sugar and crème fraiche with the whisk of the mixer until thick and creamy. Pour into a piping bag with star-shaped spout. Stir the jelly until smooth. Spray a thick cream rim onto the flat underside of about 14 Whoopies.
Add some jelly to the middle of each. Place the remaining whoopies with the flat side on top and press down gently. Dust with icing sugar.