Cream the butter. Add the eggs, one at a time, beating well with each addition. Add the sugar and vanilla sugar. Beat the batter until creamy. Combine the flour and baking powder and mix it into the batter. To finish mix in the ground hazelnuts. Leave the dough for 5-10 minutes.
In the meantime, preheat the oven (electric:175°C/convection:155 °C). Line two baking trays with parchment paper. Using an ice-cream scoop (ca. 4 cm in diameter), scoop cookies onto the baking trays, making sure the cookies have enough space between them. You can also use a pastry bag equipped with a large piping nozzle to pipe cookies on the baking sheets. Bake for ca. 15 minutes. Remove whoopies from oven and allow to cool briefly on the baking sheet, before transferring whoopies to a wire rack to cool completely.
Stiffly whip the cream and gradually add in the sugar and stabilizer. Fill a piping bag, equipped with a large star nozzle, with the whipped cream and pipe onto the underside of half the cookies. Spread the strawberry jam on the other half of the cookies, leaving a small rim. Sandwich the two cookies together and dust with icing sugar.
comments