Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides.
Step 2: In a medium bowl, combine the melted butter and brown sugar. Whisk until well combined and smooth.
Step 3: Add the egg and vanilla extract to the butter-sugar mixture. Whisk until the mixture is lightened in color and smooth, about 1 minute.
Step 4: In a separate small bowl, whisk together the flour, baking powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or spoon just until no streaks of flour remain. Be careful not to overmix.
Step 6: Fold in the butterscotch chips until evenly distributed.
Step 7: Pour the batter into the prepared baking pan and spread evenly.
Step 8: Bake for 25-30 minutes, or until the edges are golden brown and set, but the center still looks slightly soft. A toothpick inserted into the center should come out with moist crumbs, not wet batter.
Step 9: Remove the blondies from the oven and let them cool completely in the pan on a wire rack before lifting them out by the parchment paper and slicing into 16 squares.