Gather the ingredients.
Preheat oven to 350 F.
Spray a 13 x 9-inch baking pan with nonstick baking spray containing flour, or grease it using solid shortening and dust with flour.
In a heavy saucepan, melt the butter over low heat.
Remove from heat and stir in the brown sugar until blended.
Add eggs and vanilla and beat until combined.
Add the flour, baking powder, baking soda, and salt, and mix.
Stir in nuts, if using, until blended.
Spread the batter and spread it into the prepared pan.
Bake for 25 to 30 minutes or until the brownies are just set when lightly touched with your finger.
Cool on wire rack.
Frost as desired and cut the brownies into squares to serve.