Preheat Oven and Prepare Pan: Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.

Mix the Wet Ingredients: In a medium bowl, whisk together the melted butter and brown sugar until smooth. Beat in the egg and vanilla extract until well combined.

Combine and Stir: In a separate, small bowl, whisk together the flour, baking powder, and salt. Add the dry ingredients to the wet mixture and stir with a spatula until just combined. Do not overmix. If using, fold in the butterscotch or chocolate chips.

Bake: Pour the batter into the prepared pan and spread it evenly. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with moist crumbs.

Cool and Serve: Let the brownies cool completely in the pan before lifting them out using the parchment paper overhang. Cut into squares and serve.