Measure all ingredients. Sift flour, cocoa powder, and salt for the cocoa dough. Beat the egg lightly and soften butter at room temperature or microwave for 10 seconds.

Cream the softened butter and gradually mix in brown sugar in three additions.

Add half of the beaten egg and vanilla extract to the butter mixture and combine well.

Add the sifted dry ingredients to the mixture and fold using a # motion until a dough forms.

Prepare the plain dough using the same method: cream butter, add sugar in three parts, mix in egg, and fold in sifted flour.

Wrap both doughs in plastic wrap and refrigerate for at least 1 hour.

Shape the cocoa dough into 10g balls and the plain dough into 8g balls.

Flatten the plain dough with a rolling pin, place a cocoa dough ball on top, and wrap the plain dough around it. Seal the edges and gently press down with your palm.

Place the cookies on a parchment-lined baking tray and bake at 175°C (347°F) for 10 minutes.