Whisk butter, lard, sugar and salt until creamy.
Gradually stir in egg yolks.
Mix in the flour.
Place in fridge to cool for half an hour.
Form small balls and place on baking tray with baking paper.
Use a round wooden spoon handle to press holes into the balls.
Stir jam until smooth and fill into the hole. (with piping bag or freezer bag)
Bake in preheated oven at 180° C for about 18 to 20 minutes.