For the Heavy Chocolate Cookies, sift the flour, cocoa powder and baking soda into a bowl. Melt the 125 grams of dark chocolate in a water bath and let it cool briefly.
While the chocolate is melting in the water bath, beat the butter with the brown sugar on high speed for about 10 minutes until thick and fluffy. Add the egg and vanilla extract to the butter and beat for another 4-5 minutes. Then mix in the sifted flour-cocoa-soda mixture on lowest speed. Add the melted and slightly cooled chocolate and the chopped chocolate and fold in with a pastry spatula. The dough should be relatively firm.
Form about 20 balls from the dough (portion with a tablespoon) and flatten them slightly with your fingers. Place the individual cookies with enough space between them on a baking tray covered with baking paper and bake the Heavy Chocolate Cookies at 160 °C top/bottom heat for approx. 10-12 minutes.