Preheat oven to 350 degrees F. Line an 8-inch square metal pan with foil or parchment. If using foil, grease the bottom.
Put the pecan halves on a baking sheet and bake for 7 minutes or until toasty and aromatic. Let cool completely and chop. If using walnuts, you can skip the toasting.
Meanwhile, in a large (3 quart) saucepan, melt the butter over medium heat. Add the brown sugar and stir well, then remove from heat and let mixture cool for a good 15 minutes.
Stir in the vanilla and the cold egg. When egg is mixed in, stir in the baking powder and salt, followed by the flour and oats. If the batter is still warm, let it cool down a bit more before adding the chips and toasted pecans all at once.
Spread the batter in the pan and bake one center rack for 25 to 28 minutes or until bars appear brown and shiny. Let cool completely. Lift from pan and slice into bars.
Notes
Prep time includes some "waiting around" time. The mixture needs to cool before the egg is added, and again before the chips are added. If you add the chips to hot batter, they'll start to melt before you put the bars in the oven.