Start by melting 3 oz. of dark chocolate and 2 tablespoons of butter in a bowl using a double boiler or the microwave until smooth. Set aside.
Mix the dry stuff. Combine flour, cocoa powder, baking soda, and salt in a bowl. Stir until well mixed. Toss in 1 oz. of chopped dark chocolate and all the milk chocolate pieces. Set aside.
In a mixer, beat the rest of the butter and brown sugar until light and fluffy.
In another bowl, whisk eggs and vanilla together, then slowly add to the butter and sugar mixture until fluffy.
Gently fold in the dry ingredients until just combined, then add the melted chocolate and mix well.
Refrigerate the dough for at least 2 hours or overnight for best results.
Preheat the oven to 350°F and line a baking sheet with parchment paper.
While the dough chills, put granulated sugar in one bowl and powdered sugar in another.
Roll a tablespoon of dough into a ball, then roll it in granulated sugar, and then in powdered sugar until completely covered. Place on the baking sheet.
Bake for 9-11 minutes until the cookies start to crack on top.
Let cool on a wire rack.