1. Preheat the oven to 350°F. Line a 9×13-inch pan with parchment paper, allowing extra paper to hang over for easy removal, and lightly spray with cooking oil.
2. In a large bowl, whisk together melted butter and brown sugar until smooth. Let the mixture cool slightly if it’s too warm.
3. Gradually beat in the eggs, one at a time, ensuring each is fully incorporated before adding the next.
4. Stir in vanilla extract until the mixture is glossy.
5. In a separate bowl, whisk flour, baking powder, and salt.
6. Gradually fold the dry ingredients into the wet mixture, mixing until just combined to avoid tough bars.
7. Gently fold in the butterscotch chips and toffee bits.
8. Spread the batter evenly in the prepared pan.
9. Bake for 25-30 minutes until the top is golden brown and the edges are slightly darker. A toothpick inserted in the center should come out with a few moist crumbs.
10. Cool completely on a wire rack before lifting out using the parchment overhang and cut into squares.