Preheat oven to 350℉. Grease an 8x8 or 9x9 square cake pan.
In a small mixing bowl, combine flour, baking soda, baking powder, and salt. Crush the 3 graham crackers with your hands and mix them in. (You want a variety of sizes of graham pieces, so don't crush them too much.)
In the bowl of a stand mixer (or in a large mixing bowl using a hand mixer), cream together butter and both sugars until light and fluffy, about 1-2 minutes. Scrape bowl.
Add the egg and the vanilla extract. Mix until well-combined, about 1 minute. Scrape bowl.
Add dry ingredients and mix until almost combined. Add the 1/2 c chocolate chips and 1 c marshmallows, mixing until combined and all flour is mixed in.
Spread dough into your prepared cake pan. Dot the top of the bars with the 3T of peanut butter, and use a butter knife to swirl it in. It won't be a pretty kind of swirl. Pretty isn't what you're going for here - you just need to disperse the peanut butter throughout the bars.
Sprinkle the 2T chocolate chips and 1/4 c marshmallows on top of the bars.
Bake for 20-25 minutes, or until a toothpick inserted into the center of the bars comes out clean (or mostly clean with a few crumbs). Set pan on a cooling rack and allow the bars to cool completely.Cut into 16 bars and serve! Store any leftovers in an airtight container at room temperature for up to 3 days, or in the freezer for up to a month.