Mix softened butter, powdered sugar, vanilla and salt.
Add egg and mix until homogeneous.
Mix flour with baking powder.
Add the flour mixture in batches to the butter mixture and knead it briefly and quickly into a soft, roughly homogeneous dough.
Divide the dough into four equal pieces and chill them, covered, for 40 minutes.
First, take one of the four dough pieces out of the refrigerator, roll it in flour, roll it out thinly on a sheet of baking paper and cut out an 18 x 18 cm square cake base from it. Put the resulting dough scraps back in the fridge for now.
Prick the cake base several times with a fork, pull it together with the baking paper onto a baking tray and bake it in a preheated oven at 410 °F (210 °C) for about 7 minutes.
Repeat the procedure with the remaining three dough pieces and let the four cake bases cool.
Then roll out all the leftover dough thinly, bake it in the same way as the cake layers, let it cool and crumble it finely.