Mix flour and powdered sugar in a large bowl or dough bowl.

Cut the butter into smaller pieces and add them to the flour mixture.

Break the butter into the flour mixture until the butter is finely divided and resembles grated Parmesan cheese.

Combine the dough with the egg. 

Knead all the ingredients together into a uniform dough. Knead as briefly as possible; stop when the dough comes together.

Finely chop the chocolate and knead it into the dough.

Divide the dough into 3-4 equal parts.

Shape each part into an oval stick.

Wrap the dough sticks in cling film and place them in the refrigerator. 

Let them chill until they are completely cold - preferably until the next day.

Cut the dough sticks into slices about 4 millimeters thick.

Place the cookies on baking sheets lined with parchment paper.

Bake the cookies for 6-8 minutes at 200 degrees Celsius (392 degrees Fahrenheit) (180 degrees Celsius (356 degrees Fahrenheit) for convection oven).

Let the baked cookies cool on a wire rack.