Mix the butter with the powdered sugar, add the egg and mix very well. Then add the flour together with the baking powder and knead well until the dough is soft.

Make the dough into a ball and cut it in half. One half mix it with cocoa and knead it very well till all the cocoa is incorporated.

On a parchment paper, roll each ball into a rectangle and place them one on top of each other. Roll them tightly into a log and refrigerate it for about 30minutes.

Take the log out and cut in slices of less than 1cm width. Put the slices in a baking tray, on a parchment paper, few cm apart and bake them for about 20 minutes.

Then let them cool on a rack and even if they are hard, they will become softer in a few days. Enjoy!