Add the butter and dark chocolate to a microwave safe bowl. Microwave for 1 minute and stir to combine. Keep aside and allow it to cool.
In a mixing bowl, combine brown sugar, sugar and eggs. Using a hand blender, whisk together for about 3-4 minutes or until the mix is pale and the sugar is almost dissolved. The mixture should have a ribbon like consistency.
Add the cooled down chocolate mixture to the egg mixture and whip for 1 minute to combine.
Sieve in the flour, cocoa powder, salt and baking powder. Use the hand blender to whisk for 2-3 minutes or until the mixture is well blended.
Refrigerate this mix for 40-60 minutes. The dough should be set enough to make firm balls.
Preheat the oven to 180C.
Line a baking tray with parchment paper. Take about a tablespoon and a half of dough and roll it into a ball. Place these balls on the parchment paper and place the dough balls on it keeping about 1-2 inches of space between two balls as the cookies will spread while baking.
Bake for 11-13 minutes at 180C. It will still be soft to touch. Remove from the oven and let it cool for 10-15 minutes to let them set . Remove the cookies from the parchment paper and enjoy!