Melt chocolate and butter together over low heat on stovetop or microwave just until butter is melted then stir until chocolate is completely melted.
Into the melted mixture, stir sugar. Blend in eggs and vanilla, then stir in the flour, baking powder, salt, and nuts, if using.
Spread batter into a greased and floured 13x9x2-inch baking pan. Bake at 350° for 30 to 35 minutes. Cool and cut into bars.
About 24 brownies
Calories per brownie: 201