Preheat the oven to 350°F (175°C) and prepare your 9x13 inch baking pan by greasing it thoroughly with cooking spray or butter, and line with parchment paper.

Melt the butter in a small saucepan over low heat or in a microwave-safe bowl, then let it cool for about 5 minutes.

In a large mixing bowl, combine the melted butter and packed brown sugar, whisking together for about 2 minutes until glossy and smooth.

Add the eggs one at a time, whisking each until incorporated before adding the next. Then, stir in the vanilla extract.

Fold in the flour, baking powder, and salt gently until no dry flour streaks remain.

Fold in the butterscotch chips until evenly distributed.

Transfer the thick batter into the prepared pan, smoothing the top with a spatula.

Bake for 25 to 30 minutes, checking at 25 minutes by inserting a toothpick. It should come out with a few moist crumbs.

Let the blondies cool in the pan for 15 to 20 minutes before using the parchment to lift them out and transfer to a cooling rack.

Allow to cool completely for another 20 to 30 minutes before cutting into bars.