1/2 cup unsalted butter plus more for pan
1 cup all-purpose flour
1/4 tsp baking powder
1/8 tsp salt
2 oz unsweetened chocolate coarsely chopped
1 oz semisweet chocolate coarsely chopped
1 cup packed dark brown sugar
1/4 cup sugar
2 large eggs
1 tsp vanilla
1 cup pecans coarsely chopped
Preheat oven to 350 degrees. Butter an 8x8-inch baking pan. Line bottom with parchment paper, leaving an overhang on two sides. Butter paper and set aside.
In a medium bowl, whish together flour, baking powder and salt; set aside.
Place butter and chocolates in a large heatproof bowl set over (not in) a saucepan of simmering water. Heat until smooth, about 2 to 3 minutes, stirring occasionally. Remove from heat and stir in sugars, eggs, vanilla, flour mixture and half of pecans. Transfer batter to prepared pan, smooth top and sprinkle with remaining pecans.
Bake until a toothpick inserted in the center comes out clean, about 30 to 35 minutes. Set pan on a wire rack; let cool completely. Using paper overhang, lift brownies onto a cutting board. Cut into 16 squares.