Preheat the oven to 350°F/180°C. Line a 9x13 baking dish with parchment paper and spray with cooking spray, set aside.
In a medium-sized mixing bowl stir together the flour, baking powder, and salt. Set aside.
In a large mixing bowl combine the brown sugar, melted butter, eggs, and vanilla with a whisk, stirring until smooth.
Add the dry ingredients to the wet mixture and stir until there are no dry patches of flour left in the batter. Add 1 ½ cups of the butterscotch chips and fold them in until evenly distributed.
Spread the blondie batter into the prepared baking dish. The batter will be thick, so you may find it easier to wet your hands and use them to spread the batter evenly. Sprinkle a handful of butterscotch chips over the batter, saving the rest for later.
Bake for 25-30 minutes until the top appears dry and set and a toothpick inserted into the center comes out clean.
Let the bars cool completely in the pan. Then use the parchment paper to lift them out of the pan and onto a cutting board. Cut into 15 bars.
To make the topping, microwave the remaining butterscotch chips. Be sure to use a microwave-safe bowl, and cook at 15-second intervals, stirring between each, until the chips are melted and smooth. Drizzle the melted butterscotch over the bars with a spoon, or transfer it to a ziplock bag and cut off one corner to make a DIY piping bag.
Allow the topping to set for 20-30 minutes before serving.
Notes
When to add the butterscotch drizzle: I like the way these blondie bars look with the butterscotch drizzle added after they've been cut, but you can drizzle the bars before cutting them too.
Use Dark Brown Sugar: The extra molasses flavor that comes from dark brown sugar is key to getting a deep rich butterscotch flavor. Otherwise, you just have cookie bars with butterscotch chips in them.
Mix-Ins: Try adding chocolate chips, toffee bits, or chopped nuts to the batter.
To Store: Keep your blondies in an airtight container at room temperature. They will stay fresh for up to 3 days, but they're best eaten sooner rather than later.