Line a 9 by 9 square baking pan with parchment paper, making sure paper hangs over the edges. Grease paper with butter so the brownies don't stick. Melt butter and chocolate over a double-boiler and stir gently until smooth and shiny. Remove from the heat and cool slightly.
Sift together flour, baking powder, and salt into a mixing bowl and set aside. Whisk together eggs, sugar and amaretto in a mixing bowl until combined. Pour in chocolate mixture and continue to whisk until combined. Gradually add flour mixture, then add walnuts and incorporate with a wooden spoon. Transfer batter to baking pan and bake for 40 to 45 minutes or until a cake tester comes out clean when inserted into the middle of the pan. When done, invert onto a cutting board, remove paper and cut into squares. Garnish with powdered sugar and serve.