Put flour, cocoa powder, corn starch and mix it.

Stir instant coffee powder and water until dissolve

Spread butter and icing sugar toghether, put yolk in it and whisk. After that pour coffee liquid and dry ingredients continue to fold it become a dough

Using hand kneading until surface become smooth

Divided every each dough 8grams, using knife or spatula cut in the middle. Put into fridge 50min or freezer 25min

Bake in Convection oven: for 15 minutes at 140 degrees OR Conventional oven: for 14 minutes at 165 to 170 degrees.