Preheat the oven to 180 ° top / bottom heat (convection 150 °, gas level 3).
Dough:
Beat the softened butter until smooth. Stir in egg, sugar, a pinch of salt and vanilla extract. Fold in the flour and knead into a smooth dough.
Press the dough into a parchment-lined mold (22 x 22 cm) with a high rim. (Divide larger shapes with aluminum foil folded several times.)
Bake on the middle shelf for 20 to 25 minutes.
Covering:
Bring the butter, sugar, vanilla extract, a pinch of salt and the condensed milk to the boil in a small saucepan while stirring and let it boil down for about 5 minutes. Then stir in the chopped cashew nuts.
Spread the mixture evenly on the pre-baked dough and bake for another 12 to 15 minutes at the same temperature.
Let the toffees cool in the mold, then cut into approx. 60 cubes.