Ultimate Cookie Bars

Preheat oven to 375u0b0F. Spray a 9x13" pan with cooking spray.
In the bowl of your mixer, beat together butter and sugars until light and fluffy, about 3 minutes. Scrape down sides of bowl, then beat in eggs one at a time, beating for about 1 minute each until fully incorporated. Mix in vanilla.
Mix in baking powder, salt, and flour on low until just incorporated. You don't want to overmix here. Then, add in butterscotch chips, chocolate chips, and marshmallows. I usually do this by hand so the marshmallows don't get too destroyed. Make sure all are evenly distributed. If you want your bars a bit gooier, add in more marshmallows.
Plop that into prepared pan, and smooth out until even. Now, if you want, you can get crazy and add a layer of dulce de leche on top. For that, scoop the dulce into a microwave-safe bowl and microwave it for about 30 seconds or so, just so it's a bit more pliable. Dollop that on top of bars, then swirl around with a knife to distribute it around. You can do more or less here depending on your preference as well.
Bake for 20-25 minutes until edges are golden. The middle can still be slightly less done, as it will set up more when you let the pan sit out for another 10 minutes to cool. You can adjust baking time depending on your preference for gooeyness as well. The more gooey you like it, the less time you bake.