Preheat oven to 325 degrees F. Butter a 7 by 11 1/2 by 2-inch baking pan. Line with parchment paper; set aside.
In a heatproof bowl set over a pan of simmering water, combine the butter and chocolate and melt. Remove from heat and let stand until cool. Sift the flour and baking powder together into a medium bowl; set aside.
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.
Pour batter into prepared pan, and bake until top is cracked and the center is just firm to the touch, about 40 to 45 minutes. The brownies should not have a cake-like consistency. Remove from oven and let cool in pan.
Cut into squares in the pan before turning out onto a board.
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