Ensure butter is in room temperature. Mix butter, sugar, (and vanilla extract), until smooth and creamy.
Add in egg yolk and mix until well incorporated.
Add flour (and cocoa powder). Mix and gently knead the dough till smooth out.
Refrigerate the cookie dough for several hours before use.
To Bake : Reheat oven at 180 Degree Celsius. Shape cookies using Fusion Master Cookie Cutter and bake the cookies into the oven for 13 mins (standard cookie size), or another 2 to 3 mins (larger cookies)