Preheat the oven to 180°C.
Rub a baking tray with butter.
Be sure that the butter is soft.
Add the sugar.
Add the egg and keep on stirring.
Sieve the flour into the bowl and mix well.
Pour the dough onto the baking tray.
Be sure that it’s very thin.
Bake it for about 15 minutes at 180°C.
Remove from the oven immediately.
Cut into 32 pieces.
While the cookies are in the oven you can make
Mix the butter with the sugar.
Add one egg yolk.
Beat this very well until smooth.
Put the batter in a piping bag.
Pipe the spread onto half of the cookies.
Place the other halves of the cookies on top.
Finish with a bit of the spread on top.