Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper and spray the paper with cooking spray. Set it aside.
In a medium-sized bowl, whisk together the flour, baking powder, and salt until well combined and set aside.
In a large bowl, whisk together the brown sugar, melted butter, eggs, and vanilla until smooth.
Add the dry ingredients to the wet and stir them together until just combined. Add 1-1/2 cups of the butterscotch chips and fold them in until evenly distributed.
Spread the batter into the prepared baking dish. Take a handful of the remaining butterscotch chips and sprinkle it over the batter for a pretty presentation.
Bake for 25-30 minutes until the top appears dry and set and a toothpick inserted into the center comes out clean.
Let them cool completely in the pan. Use the parchment paper to help you lift the bars out of the pan. Cut into bars.
Place the remaining butterscotch chips into a microwave-safe bowl. Microwave in 15-second intervals, stirring in between until melted. Place the melted butterscotch into a small zip-top bag and cut a small piece of the corner off. Drizzle the melted butterscotch chips over the cut bars. Let them set for about 20 minutes to allow the butterscotch drizzle on top to solidify.
Serve immediately.
Notes
I find it easier to spread the prepared dough into the pan with moistened hands.
I think the bars look nice with the butterscotch drizzle on top after cutting into bars, but if you want to drizzle it over the entire pan before cutting, you can do that too.
You can leave off the drizzle altogether. It's entirely up to you.