Preheat oven to 350 F degrees F. (176 C). Line a 13×9 inch pan with foil or parchment paper or foil. I used foil and greased only the bottom.
Mix together the flour, baking powder and salt; set aside.
Mix together butter and brown sugar. If you’ve used very hot butter, let the mixture cool for 5 minutes before beating in the egg. With a spoon or whisk, gently beat in the eggs, vanilla and rum. Stir dry ingredients into batter.
Let the batter cool down if it is still warm, then stir in the chips and walnuts (optional).
Spread in pan and bake for about 35 minutes or until pale golden and edges start to pull away from the sides. Cool at room temperature or quick cool in an ice bath. I used the ice bath.
