Preheat oven to 350°. Grease a 9″x 13″ baking pan. Line bottom of pan with parchment paper.
Melt butter, half of the chocolate chips, and usweetened chocolate in a double boiler (or microwave). Let mixture cool to warm.
Whisk sugar, eggs, and vanilla just until mixed. Over-mixing will cause crumbly brownies. Fold in warm chocolate mixture.
Whisk flour, baking powder, baking soda, and salt. Fold dry ingredients into chocolate mixture until just combined. Stir in remaining chocolate chips and nuts (if using), mixing just until combined.
Pour batter into prepared pan. Spread to an even thickness. Bake 32-36 minutes. The edges will be slightly firm and the center will be soft when they are fully baked. Do not overbake. Cool completely in pan on wire rack.
Makes 12 large or 24 small brownies. These brownies may cut better if they are chilled first.