Preheat oven to 350 degrees F. Line a rimmed 9x13-inch baking sheet with parchment paper.
Break the semisweet and unsweetened chocolate into 1-ounce pieces. Bring the butter to a boil in a heavy-bottomed saucepan. Add both chocolates and immediately remove from the heat. Stir until the chocolate melts completely.
Combine the flour, baking powder, and salt in a small bowl and set aside.
Combine the eggs and sugar in the bowl of the electric mixer. Beat until the mixture is very thick and forms a ribbon when you lift the beaters away from the batter. Add the vanilla and blend lightly. Add the melted chocolate and mix lightly.
Gently fold the flour mixture into the chocolate batter with a spatula. Fold in the chocolate morsels and nuts.
Spread the batter in the greased baking pan. Bake for 35 to 40 minutes. When done, a cake tester inserted into the center of the brownies will have a little batter clinging to it. Do not overbake the brownies. They are super-delicious when a little wet and "gooey.".
Cool the brownies in the baking pan.
Cut the brownies into 1-1/2-inch squares. Serve with fresh fruit, or topped with ice cream for a sinfully rich brownie sundae.