Whisk the flour, baking powder, and salt together in a small bowl to aerate and combine; set aside.

Melt butter over medium heat in a medium saucepan or in a large microwave safe bowl. If you use a saucepan, pour melted butter into a large mixing bowl or if you used the microwave you can proceed using the same bowl. Cool slightly until warm. Whisk in the brown sugar and vanilla until combined, then whisk in the egg. Fold in the flour mixture until a few floury streaks remain, then fold in chocolate chunks and nuts just until combined. Scrape into prepared pan and smooth top with a small offset spatula.

Bake for 20 to 25 minutes or until a toothpick inserted in the center shows a few moist crumbs clinging when removed. The bars should be light golden brown, slightly puffed, and the edges will have just begun to come away from the sides of the pan. Cool in the pan on a rack for 10 minutes, pull up on foil or parchment to remove bars from pan, then cool bars on rack completely. Peel away foil or parchment and cut into a 5 by 5 grid (25 bars). Store at room temperature for up to 3 days in an airtight container in single layers separated by aluminum foil parchment paper. Individually wrapping the bars will keep them as fresh as they can be. Bars can also be individually wrapped and frozen for up to a month, which makes them very easy to grab on the go. They defrost very quickly.