Adjust the oven rack to the 2nd level position just above center and preheat then oven to 350ºF.
Generously spray an 8 inch square pan with a flour-based baking spray. Set aside. Tip: If you’d like to be able to pull the brownies out of the pan easier to cut them, add 2 overlapping strips of parchment paper on top of the flour-based coated pan, then spray again with the flour-based baking spray. If you plan to just keep them in the pan to cut them, then you can skip the parchment paper.
Using a food processor, pulverize the graham crackers into fine crumbs. Add in the melted butter and pulse until fully combined.
Spoon the graham cracker crust into the borrow of the prepared 8” x 8” brownie pan, then place in the freezer to chill while you prepare the brownie batter.
In a medium bowl, whisk together the all-purpose flour, salt, baking powder, and unsweetened cocoa. Set aside.
Using your hand-held mixer or stand mixer fitted with a paddle attachment, beat the unsalted butter, vegetable oil, and granulated sugar together on medium speed until well blended.
Add in the slightly whisked eggs and vanilla extract, scraping down the sides of the bowl as needed.
Switching the mixer speed to low, add in your dry ingredients a little at a time until it’s blended with the wet mixture. Be careful not to over mix the brownie batter.
Bring the pan out from the freezer and spoon half of the brownie batter onto the top of the chilled graham cracker crust. Evenly place each unwrapped Ghirardelli Milk Chocolate Caramel Square in a grid pattern, then pour the remaining half of brownie batter over top.
Bake for 35-40 minutes at 350ºF. Avoiding the caramel squares as best you can, insert a toothpick to make sure your brownies are done. If they still have crumbs on the toothpick, remove them from the oven. If they still have brownie batter on the toothpick, then keep them in, but monitor them every minute until done.
Using hot pads move the oven rack from the 2nd level position to the 3rd level position. Switch your oven settings from bake to broil, and set to HI.
Carefully top the brownies with the mini marshmallows, then place the pan into the broiler.
Toast/broil the marshmallows for 3-6 minutes max. Closely monitor this part of the recipe carefully so the marshmallows do not burn.
Remove from the oven when the mini marshmallows have puffed up and have a very light golden brown color to them.
Remove s'mores brownies from the oven and allow to cool completely before cutting into them.