Preheat the oven to 350 degrees. Spray a 9 x 9, 8 x 10, or 9 x 13-inch baking dish with baking spray. (Or you can line with aluminum foil.)
***Note: If you use a rectangular pan, you may need an additional can of dulce de leche to cover the surface.
Mix together the flour, oats, brown sugar, baking powder, and salt. Cut in the butter with a pastry cutter until it resembles coarse crumbs. Sprinkle the mixture into the pan and pat lightly to pack it slightly.
Bake until light golden brown on top and done in the middle, about 30 to 35 minutes, watching to make sure it doesn't burn. Remove from the oven and allow to cool for 15 minutes.
Spoon the dulce de leche into a microwave safe bowl and nuke it for 45 seconds, just to slightly soften it. Scoop it on top of the oatmeal base and use an offset spatula to spread it out to the edges and into an even layer. Sprinkle on a layer of peanuts so that they completely cover the caramel, and use your hands to gently press them into the caramel. Sprinkle on the butterscotch chips and the mini chocolate chips in generous layers.
Note: The warmth of the cookie base and the caramel should slightly soften the butterscotch and chocolate chips. When that happens, use your hands to very gently press the chips just enough to anchor them together (but not enough to misshape them.) If the pan isn't warm enough, pop it into the oven for 30 seconds or so and gently press the chips to anchor them together.
Chill the bars for 2 to 3 hours to make them easy to slice. Turn them out onto a cutting board and use a long serrated knife to cut into small squares (they're rich!)
Serve cold or at room temperature.