Preheat the oven to 350°F. Lightly grease a 13 x 9-inch baking pan and line it with parchment paper; set aside.

In a large bowl, combine 1 cup unsalted butter (melted and cooled slightly), 1 ⅔ cups brown sugar (packed), and ⅔ cup granulated sugar. Stir until smooth and combined.

Add 2 eggs and 1 egg yolk (both at room temperature) and 1 tablespoon vanilla extract to the bowl. Stir until the eggs are fully incorporated and the mixture is uniform.

In a separate bowl, whisk together 2 ¼ cups all purpose flour (spooned and leveled), 2 tablespoons corn starch, 1 teaspoon baking powder, and 1 teaspoon salt to evenly distribute the dry ingredients.

Add the dry ingredients to the wet ingredients and stir just until combined—mix only until no large streaks of flour remain to avoid overmixing.

Fold in 1 cup butterscotch, peanut butter, semi-sweet, or white chocolate chips (your choice) until evenly distributed.

Transfer the batter to the prepared baking pan and spread it into an even layer with a spatula.

Bake for 22 to 27 minutes, checking near the edge with a toothpick; the blondies are done when a toothpick inserted near the edge comes out with just a few moist crumbs.

Allow the blondies to cool completely in the pan on a wire rack before lifting out (using the parchment) and cutting into bars.