1. Preheat oven to 325°. Line brownie pan with parchment paper; set aside.

2. Combine the butter and chocolate in a heatproof bowl set over a pan of simmering water, and melt. Remove from heat, and let stand until cool. Sift the flour and baking powder together into a medium bowl; set aside.

3. In the bowl of an electric mixer fitted with the whisk attachment, beat the eggs until thick and creamy, about 5 minutes. Gradually beat in the sugar. Continue beating until mixture is thick and mousse-like and a trail is left on the surface when the whisk is lifted, about 3 minutes more. Gradually fold in the chocolate mixture, alternating with the dry ingredients. Fold in the nuts.

4. Pour batter into prepared pan, and bake until top is cracked and center is just firm to the touch, 40 to 45 minutes. (The brownies should not have a cakelike consistency). Remove from oven, and let cool in pan.

5. Cut into squares in the pan before turning out onto a board.