Preheat oven to 350 degrees F. Line an 8 inch metal pan with foil. Spray bottom only with cooking spray. Alternatively, you can use parchment paper.
Melt the butter over medium heat in a small saucepan. Turn heat to low, add chocolate, stir for a second, then turn off the heat and melt chocolate with residual heat. Let cool for 10 minutes.
Add the sugar to the cooled chocolate mixture and stir well, then add each egg, one by one, whisking or stirring until blended. Whisk in vanilla.
Mix together the cake flour, baking powder and salt and add to batter. Stir until blended. If using nuts, add them too.
Pour the batter in the prepared pan. If desired, sprinkle chocolate chip over the top (and maybe some extra nuts!). Bake on center rack for about 20 minutes or until brownies start to smell done and look set. They'll puff up a little in the center and sink a bit as they cool.
Let to room temperature or quick cool by setting the pan in a larger pan of ice water. Lift foil from pan when brownies are cool. Cut into whatever size you like.