Preheat your oven to 350°F (175°C) and line an 8-inch square baking pan with parchment paper.

In a medium-sized bowl, whisk together melted unsalted butter with both light brown and granulated sugar until glossy and smooth.

Beat in the eggs one at a time and stir in the pure vanilla extract until smooth.

In a separate bowl, whisk together the all-purpose flour, baking powder, and kosher salt.

Gently fold the dry ingredients into the wet mixture until no visible flour.

Fold in the butterscotch chips and optional chopped nuts.

Transfer the thick batter into the prepared baking pan, smoothing the top.

Bake for 23-27 minutes until edges are set and center remains slightly moist.

Cool completely in the pan on a wire rack, then lift out and slice into squares.