Preheat the oven to 175 degrees. Put the baking sheet on parchment paper. Using a mixer, combine the butter and sugar, whisking until light, fluffy. Add the eggs one at a time, then add the cocoa powder.
Add flour, salt and baking powder. Add pieces of chocolate and mix gently.
The dough will be very thick. Knead it lightly with your hands and divide into 12 parts. Roll each part of the dough into a ball and slightly press down. If desired, decorate the top with drops of chocolate.
Put cookies on a baking sheet and bake for 20 minutes. Allow to cool on a baking sheet for 5 minutes, then cool completely on a wire rack.