Work the butter, sugar and flour together with a mixer.
Add egg yolks and vanilla sugar and continue mixing.
Mix until all combined.
Remove from the mixing bowl and knead by hand to form one solid shape.
Transfer dough to a plastic bag and place in the fridge for 30 minutes.
In the meantime, wash and dry the mixing bowl well.
Add 3 egg whites with a pinch of salt and mix on medium speed for 7 minutes.
When bubbles start to form, gradually add sugar (spoonful by spoonful).
Continue mixing until egg white peaks.
Remove dough from fridge and bag and place on parchment paper.
Begin rolling with a rolling pin. Works best with another sheet of parchment paper on top.
Roll until 5mm thickness.
Preheat the oven to 320F.
Use a round cookie cutter about 1” diameter.
Dip the edges of the cutter in four and cut rolled out dough.
Transfer circle shapes onto a parchment lined baking sheet.
Bake for 7-10 minutes.
Remove from the oven.
Add egg white mixture into a piping bag and squeeze out the mixture onto each of the baked circles.
Put back in the oven to bake for another 15 minutes.
Remove from the oven and let cool.
Spread a layer of jam (such as strawberry or peach jam) on the flat side and stick flat ends together to form the complete cookie.
