In microwave oven, on medium power, melt butter and chocolate in large microwave-safe bowl, about 3 minutes. Stir until smooth. Mix in granulated sugar and brown sugar; then beat in eggs, one at a time, with wire whisk. Add vanilla extract and blend. Stir in nuts. Combine sifted flour, baking powder, and salt and fold into chocolate. Spread batter in 9x13" pan that has been greased on bottom only. Optional: Sprinkle the 1 and 1/2 cups of chocolate chips on top of the batter. Bake in preheated oven at 350 deg. for about 33-35 minutes. Do not over-bake; toothpick may have some crumb on it, but should not be wet.