Preheat oven to 350 degrees
Line 9x9 pan with parchment paper - set aside
Melt butter in a large bowl in microwave for approx 25-40 seconds
Whisk in peanut butter & brown sugar until fully combined
Whisk in eggs
Switch to a large spatula or wooden spoon & stir in salt, flour & baking powder
Transfer half the batter to prepared pan & press into a thin, even layer
Top with Oreo cookies & sprinkle in white chocolate & peanut butter chips
Add remaining batter, pressing into a flat layer - it's okay for some of the Oreos to show though, just not all of them.
Bake 20 minutes - then (using hot pads) carefully cover pan with foil & bake an additional 15-18 minutes
Remove foil & bake another 5 minutes or until tops no longer look doughy
Let cool in pan until ready to serve