In a heat safe bowl, combine the dark chocolate and butter. You can use a microwave for 30 second intervals, stirring thoroughly and completely before moving on or you can place the bowl above a fitted stock pot with 1-2 inches of water in the stock pot below. Keep the heat on low and stir the chocolate and butter thoroughly.
Whichever method you choose, set the chocolate and butter aside once melted. Allow the chocolate and butter to cool to near room temperature so that it doesn't melt the sugar.
Preheat the oven to 350°F. Line a large baking sheet with parchment paper.
In a large bowl combine the white sugar, brown sugar, egg and vanilla.
Use a mixer to beat the mixture on high for 7 minutes allowing the volume and texture to change.
Gently mix in your chocolate and butter mixture.
Sprinkle in the cocoa powder, salt, baking powder, and flour. Mix gently only mixing until the ingredients are incorporated.
Mix in your chopped milk chocolate until evenly incorporated.
Immediately scoop the dough into approximately 2 tablespoon sized dough balls.
Bake for 10-12 minutes. Look for crinkly tops to know when they are done.
Remove the cookies from the oven but leave them on the baking pan for at least 5-10 minutes.
Optional: While still warm, dust them with a touch of flaky sea salt.