8x8 or 9x9 pan (8x8 for thicker bars; 9x9 for thinner bars; we like 8x8 best!)
3/4cupunsalted butter,partially melted and cooled
3/4cup (150g)light brown sugar, packed
1cup (135g)white flour
1cup (102g)old fashioned oats,do not use quick or steel oats
1bag (12 ounces)milk chocolate chips
1jar (7 ounces)marshmallow fluff(I use Jet-Puffed Marshmallow Creme)
PREP: Preheat the oven to 350 degrees F. Line an 8x8 or 9x9-inch baking pan with parchment paper and set aside. We prefer 8x8 pan for thicker bars, but either pan size works.
COOKIEBASE: In a large bowl, mix the 3/4 cup melted and cooled butter (See Note 1), 3/4 cup brown sugar, 1 cup flour, 1 cup oats, 1 teaspoon baking soda, and 1/2 teaspoon salt. Stir until combined into a thick cookie dough. Divide the mixture in half and press one half of it into the prepared pan.
BAKE: Bake at 350 degrees for 8 minutes, then remove.
CHOCOLATE AND MARSHMALLOW: Immediately out of the oven, sprinkle the 1 bag of chocolate chips evenly over the cookie crust. Warm the marshmallow fluff in the microwave (make sure there is no metal/foil topping on the fluff or don't microwave) for 7 seconds. Spoon the marshmallow fluff on top of the chocolate chips and, working carefully (and slowly), gently smooth into one layer.
REMAINING COOKIE DOUGH: Crush up the 2 graham crackers (I put them both in a bag, seal the bag, and crush with a wooden spoon or handle of a knife; just a coarse crush is enough). Add the crackers to the rest of the cookie dough mixture and stir to combine. Crumble the cookie dough mixture over the marshmallow fluff. Bake for another 13-15 minutes until the top layer is lightly browned.
COOL: Remove from the oven and allow to cool at room temperature for at least 1 hour before cutting into the bars. Using the overhang of parchment paper, pull bars out and cut with a sharp knife. Best enjoyed within 2-3 days (store in an airtight container).
Note 1: Don't add hot butter to the sugar -- it will melt the sugar and cause greasy bars.