Heat oven to 350°F. Line a 9x13-inch pan with parchment and lightly grease.
Whisk melted butter with brown sugar and granulated sugar until glossy and smooth.
Whisk in the beaten eggs and vanilla until the mixture looks slightly thickened.
Combine flour, baking powder, baking soda, and salt in a separate bowl.
Stir the dry ingredients into the wet just until no dry streaks remain.
Fold in the chocolate chips and walnuts, if using.
Spread batter evenly in the pan and smooth the top. Sprinkle mini chips over.
Bake 22–26 minutes, until edges are set and a tester shows a few moist crumbs.
Cool the pan on a rack. Lift out with parchment and slice into squares.