Prepare your baking space. Preheat your oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease any exposed pan areas.
Melt the butter in a medium saucepan over medium-low heat. Once melted, remove from heat and immediately add the brown sugar. Whisk until well combined and slightly cooled, about 2 minutes.
Add eggs one at a time, whisking well after each addition until fully incorporated. The mixture should become smooth and slightly lighter in color. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt. Add this dry mixture to your wet ingredients and fold gently with a rubber spatula just until no dry streaks remain. Do not overmix.
Fold in butterscotch chips and nuts if using, distributing them evenly throughout the batter. The batter will be thick and somewhat sticky.
Transfer the batter to your prepared pan and spread it evenly with a spatula, smoothing the top. Give the pan a gentle tap on the counter to release any air bubbles.
Bake for 22-25 minutes until the edges are set and lightly golden, but the center still appears slightly underdone. A toothpick inserted should come out with a few moist crumbs (not wet batter).
Allow to cool completely in the pan on a wire rack before lifting out using the parchment overhang. For the cleanest cuts, refrigerate for 30 minutes before slicing into 16 squares.