Melt the unsweetened and bittersweet chocolates together and set aside
Place the butter and sugar in the bowl of a stand mixer and mix until combined. Add the eggs 1 at a time. Add in the flour, baking powder and salt and mix just until combined. Add the melted chocolate. Stir in the walnuts and chopped chocolate pieces.
Spoon small mounds of the dough onto a parchment paper lined baking sheet. If you want to make giant cookies, use an ice cream scoop instead. Space the dough mounds evenly and bake in a preheated 350 degrees F convection oven for 16 minutes.