1. Beat butter and sugar in a mixing bowl for a few minutes until creamy.
2. Add egg, beat well on medium speed until well combined.
3. Add sifted flour, cocoa powder and baking soda followed by almond flakes. Mix well on slow speed for a minute.
4. Divide the dough into 2 equal parts and place each dough onto a plastic sheet. Using a rolling pin, press it into a rectangular block about 2.5 cm thick. Leave it in the freezer to harden for an hour.
5. Remove the blocks from the freezer and cut into small rectangular pieces (2.5cm x 4cm), about 0.5cm thick.
6. Bake in a preheat oven at 180 degree Celsius for about 15 minutes.