Preheat oven to 350°F/175°C. Line a 8x8-inch square pan with parchment paper, leaving overhang on the sides for lifting the bars out of the pan after baking.
In a medium bowl whisk together flour, baking powder and salt. Set aside.
In a large bowl mix melted butter with granulated sugar and brown sugar until smooth and shiny. Whisk in the eggs and vanilla until they're just barely combined, don't overmix. Add in melted butterscotch and whisk until combined and smooth.
Add flour mixture to wet mixture and fold, using a rubber spatula or wooden spoon, just until combined. Gently fold in butterscotch chips until evenly distributed.
Pour the batter into prepared pan and smooth the top. Bake for 20-22 minutes, until the top is lightly golden brown and slightly firm to touch. The middle will still be undercooked, but they will continue to cook as they cool. Allow to cool to room temperature on a wire rack, then lift out of the pan and cut into squares. Serve warm or at room temperature.
Store bars in an airtight container at room temperature for up to 3 days, or freeze for up to 2 months.