Grease a square springform pan (24 x 24 cm; alternatively round, 26 cm Ø) and dust with flour. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Mix 500 g flour and sugar in a bowl.
Add vanilla flavouring and 350 g butter in pieces. Knead first with the dough hooks of the mixer, then with your hands briefly to crumble. Scarcely 3⁄4 Put the crumbles into the form and press them to a base.
Stir red fruit jelly until smooth. Spread on the crumble base, leaving an approx. 1/2 cm wide rim free. Spread the rest of the crumble on the groats. Bake in the hot oven on the second shelf from the bottom for about 45 minutes.
Remove from the mould and let it cool down in the mould. Remove the cake from the edge with a knife and cut in half. Cut each half crosswise into about 15 strips. Serve with vanilla sauce.