Preheat oven to 350°F (175°C). Line a 9"x 13"x 2" baking pan with parchment paper so that it overhangs by a couple of inches on the two long sides. Grease the parchment and any exposed parts of the inside of the pan.
Whisk together the flour, baking powder, and salt. Set aside.
In a separate bowl, combine the butter, brown sugar, eggs, and vanilla until thoroughly combined. Gradually add the flour, mixing just until combined. Stir in the butterscotch chips and nuts.
Spread the batter evenly in the prepared pan. Sprinkle a few pinches of coarse salt over the top of the bars.
Bake 30 to 35 minutes, or until the bars are golden brown and a pick inserted into the center comes out clean.
Place the pan on a wire rack and allow the bars to cool completely. Use the overhanging parchment paper to lift the bars from the pan and place on a cutting board. Cut into bars.
Notes
Store in an airtight container at room temperature up to 4 days.