Preheat the oven to 325°F, and prepare a 9 x 9-inch baking pan by spraying it lightly with cooking spray or lining it with a piece of parchment.
Melt the butter in the microwave for 45-60 seconds. Add the brown sugar, eggs and vanilla extract. Stir together until everything is combined.
Add the flour, baking powder, and salt. Stir until everything is well combined.
Fold in half of the chocolate chips (and walnuts, if desired). Pour the batter into the prepared baking pan, smoothing it out to the edges. Sprinkle the remaining chocolate chips and walnuts on top of the dough.
Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.