Leave butter at room temperature
Separate egg yolk and whites (won’t use the whites)
Sift sugar, flour, coco power separately. Set aside.
Separate flour into two parts. Weigh one to 220g and another one to 200g.
Mix butter with electric mixer until butter’s color changes to white and texture should be creamy. 5 minutes.
Add sifted sugar and mix with mixer well. 3 minutes.
Add vanilla essence and mix with mixer, 1 minute.
Add egg yolk and mix with mixer well, 3 minutes.
Separate the mixture evenly into two bowls.
In one bowl, add 220g flour. Another one add 200g flour and cocoa powder.
Start from the white one. Mix it well with a spatula until it becomes a crumbling texture. Then use your hand and form into a 2 inch diameter block shape. Repeat it for the cocoa powder one.
Wrap blocks with plastic wrap. Keep them in the refrigerator for 2 hours.
Bake them at 350 degrees for 18 minutes.