Spray a 10-inch cast iron skillet with baking spray - the type of spray that contains flour. If you don't have any, grease the skillet with butter, then dust with flour. Or simply use my Baking Spray Replacement. (Linked in the post above.)
In a bowl, stir together the flour, baking powder, and salt. Set this aside.
In your mixing bowl, beat together the butter and sugar until fluffy.
Beat in the egg and vanilla extract.
Add half of the flour mixture to the butter mixture and mix until combined.
Stir in the milk.
Add in the remaining flour mixture, mix until combined. Scrape the bowl with a rubber spatula to make sure everything is combined.
Add in the oats, chocolate chips, and pecans. Stir until combined.
Spread half of your dough into your prepared skillet. Press it into the skillet. This works best if you wet your hands then press the dough down. The dough will not stick to your damp hands.
Spread the peanut butter over the top of the dough.
Carefully spoon the remaining dough over the top and press id down again with damp hands.
Bake this for 35 minutes, uncovered. Loosely cover the skillet with foil and bake for an additional 10 minutes.
If you are serving this as a dessert, allow it to cool for 20 minutes before serving with ice cream. If you have prepared these as snack bars, allow it to cool completely before cutting. These bars are moist, rich, and tender.