Prepare all the ingredients.

In a bag, mix butter and sugar thoroughly. Add the egg and continue mixing until sugar dissolves.

In another bag, combine flour, cocoa powder, and chocolate chips. Shake well.

Combine the contents of both bags and knead into a dough.

Divide the dough into two portions and shape them into desired forms (e.g., rectangular or cylindrical).

Freeze the dough for at least 1 hour.

Slice the frozen dough into 0.5cm thick pieces. Place them on a baking sheet lined with parchment paper.

Bake the cookies on low heat for 12 minutes with the lid on. Flip them and bake for another 5 minutes.