Preheat oven to 350°F. Line a 13x9-inch baking dish with parchment paper leaving a 2-inch overhang on all sides. Spray paper with cooking spray, set aside.
Bars: In a medium-size mixing bowl use a whisk to sift together the flour, baking powder and salt until combined.
In a separate large mixing bowl, use a whisk to stir together the brown sugar, melted butter, vanilla and eggs until smooth.
Add the dry ingredients to the wet mixture stirring together until fully combined. To the batter add 1 ½ cups butterscotch chips. Stir to distribute.
Spread batter evenly in the prepared pan. (It will be thick) Sprinkle the top with 1/2 cup reserved butterscotch chips.
Place into the oven and bake for 25-30 minutes r until the top appears dry and set and a toothpick inserted into the center comes out clean. Cool completely in the pan on a cooling rack.
Butterscotch Glaze: Place the remaining 1/2 cup butterscotch chips and heavy cream in a microwave-safe bowl. Microwave in 15-second intervals, stirring in between repeating this process until they're completely melted and smooth.
Lift bars from pan using the parchment paper "handles". Cut into desired size bars. (You can slice into 12 or 16 bars.)
Pour melted butterscotch into a piping bag or a plastic Ziplock bag with the corner snipped off. Drizzle the glaze over the cut bars, let them dry for about 20 minutes to set, then serve.
Notes
Alternate Mix-Ins: You may like to swap out some of the butterscotch chips with milk chocolate chips, semi-sweet chocolate chips, white chocolate chips, chopped-up peanut butter cups, toffee bits and peanut butter chips.
Nuts: You may like to add 1 cup of chopped pecans or walnuts to the cookie dough.
Butterscotch Drizzle Substitution: Some other topping options you can use instead of using more butterscotch chips would be melted milk chocolate chips, semi-sweet chocolate chips, white chocolate chips, or dark chocolate chips. You can also leave off the topping altogether, it's entirely up to you.
Frosting: You could also frost the top with chocolate buttercream, melted creamy peanut butter or brown sugar frosting.
Brown Sugar: You can use light brown sugar in place of dark brown sugar. Doing so, will make the texture more like butterscotch blondies.