Knead the butter in pieces, sugar, 200 g flour and egg first with the dough hooks of the mixer, then quickly with your hands to a smooth dough. Cover and chill for about 30 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Slightly flour a sheet of baking paper, roll out the dough 4-5 mm thin on top.
Spread half of the pastry sheet with jam. Fold over the other half of the dough using the paper. Pull the dough sheet with the paper onto a baking tray. Prick the surface several times with a fork.
Bake in a hot oven for 20-25 minutes. Take out and cut into approx. 60 strips (approx. 1 x 6 cm) while still hot. Leave to cool completely on the tray.