Butter a 12x18 jelly roll pan or a baking sheet with sides. In a saucepan, combine the butter and cocoa powder over medium heat, stirring until melted and combined. In a large bowl, combine the sugar and eggs. Add the cocoa mixture and mix gently. Into another bowl, sift the flour, baking powder and salt and add a little at a time to the wet ingredients, mixing until incorporated. Fold in the nuts. Pour batter into the prepared pan and bake for about 30 minutes, checking periodically until a toothpick inserted into the center comes out clean. Let cool, then cut into squares or bars.