Preheat the oven to 350°F. Line a 9×13-inch metal baking pan with parchment paper, leaving overhang on two sides. Lightly grease.
In a large bowl, whisk the melted butter and brown sugar until smooth and glossy, about 1 minute.
Whisk in the eggs and vanilla until fully combined and slightly thickened.
In a separate bowl, whisk together the flour, baking powder, and kosher salt.
Add dry ingredients to the wet ingredients and fold just until no dry streaks remain. Do not overmix.
Fold in the butterscotch chips and pecans (if using).
Scrape the thick batter into the prepared pan and smooth the top. Sprinkle with flaky sea salt if desired.
Bake for 22 to 26 minutes, until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs.
Cool completely on a wire rack. Use the parchment overhang to lift out, then cut into 16 bars. Store airtight at room temperature for up to 3 days.
Notes
For clean slices, chill the pan for 20 minutes before cutting. Swap pecans for walnuts or omit for nut-free bars. Slight underbaking keeps the center fudgy; bars continue to set as they cool.