Preheat the oven to 350F/ 177C. Spray a 9x13-inch baking pan with baking spray and line it with parchment paper going up the sides for easier removal.

In a medium bowl, whisk together the melted butter, sugars and salt until blended. Add the eggs and vanilla and whisk until just combined.

Switch to a spatula, and add the flour, baking powder and baking soda and fold it all together until almost fully combined. Then add the candy pieces and continue folding until combined, but do not over-mix.

Pour the batter into the prepared pan and spread evenly. (You can add a few more candy pieces to decorate the top if desired.) Bake for about 25 minutes or until a tester inserted into the center comes out almost clean. (Slightly under bake for gooey brownie-like bars or a little longer for cake-like bars.)

Set the pan on a wire rack to cool. Then cut into squares and serve.