Mix the butter, oil, vinegar, sugar, salt, baking powder, egg white, and flour in a bowl.

Knead together (works both with your hands and with a hand mixer).

Put the dough for at least half an hour into the fridge.

Take the dough once it is rather solid. Roll it on a floured surface.

Preheat oven to 400 °F/200 °C.

Cut out the cookies and put them on a sheet.

Whisk the egg yolk and spread it on the cookies. Add the sesame and black seeds to taste.

Put the cookies into the oven and bake for about 15 minutes until they turn brown.