Begin by preheating your oven to 350°F (175°C). Prepare a 9x13 inch baking pan by greasing it with butter or lining it with parchment paper.
In a large mixing bowl, add the melted butter and packed brown sugar. Using a whisk, blend these ingredients together thoroughly until you have a smooth and glossy mixture.
Introduce the eggs to the buttery mixture, adding them one at a time. Be sure to whisk well after each addition, then mix in the vanilla extract.
In a separate bowl, combine the all-purpose flour, baking powder, and salt. Using a whisk, mix them well. Gradually fold the dry ingredients into the wet mixture using a spatula or wooden spoon.
Carefully fold in the butterscotch chips and chopped pecans (if using) until they are evenly scattered throughout the batter.
Pour the batter into the prepared baking pan, spreading it out evenly.
Place the pan in the preheated oven and bake for 25-30 minutes until the edges turn golden and a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, let the blondies cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.
Once fully cooled, cut them into squares to serve.