Pre-heat oven to 325F degrees. Place oven rack at middle position. Butter and flour an 8" cake pan (square or round). Set aside.
Melt chocolate and butter, using your favorite method (in the microwave or on the stove) and set aside to cool.
In a medium bowl, beat the eggs until light and fluffy. Add sugar and beat until creamy.
Add the melted chocolate mixture and mix for about 30 seconds.
Whisk baking powder into flour and add to wet ingredients. Stir just till incorporated and batter is all one color. Don't over stir.
Add nuts, if desired and pour into 8-inch square or round cake pan.
The batter will be quite thick. Spread almost to edge of pan with a rubber spatula.
Bake 22-25 minutes or until a toothpick comes out with wet crumbs. Don't over cook the brownies.
If you want to remove the brownies from the pan, cool for 10 minutes, then cover with the wire rack and invert, tapping a bit to release the brownies. Then gently cover the brownies with serving plate and invert again. Cut brownies into serving size pieces and dust with powdered sugar if desired.