Preheat the oven to 300°F (150°C).

In a bowl, mix together the flour, cornstarch, and baking powder. Set aside.

In the bowl of your stand mixer, add the softened butter, powdered sugar, vanilla sugar, salt, and vanilla extract. Mix on high speed until well combined.

Add the egg yolk, then mix in the flour, cornstarch, and baking powder mixture on low speed until just combined.

Divide the dough in half. Shape one half (vanilla) into a log and set aside. Add the cocoa powder to the other half and mix quickly until the cocoa is fully incorporated.

Bake for 15 minutes.

Enjoy!