Preheat oven to 325ºF and grease a mini 6″ cast iron skillet well with butter or oil, set aside.
In a medium bowl stir together melted butter and sugar until well combined. Add in egg yolk and vanilla; stir until evenly mixed.
Add in flour, cocoa powder, baking soda and salt; stir with fork or spatula to combine. Add in milk one tablespoon at a time until batter is thick but somewhat smooth.
Fold in crushed graham cracker and chocolate chips.
Bake for 9-10 minutes, making sure not to over-bake.
Remove skillet from oven and place marshmallows cut side down on top of brownie. Change oven to broil (about 400ºF if possible) and brown marshmallows slightly for about 90 seconds – watch marshmallows for the 90 seconds to make sure they don't burn.
Notes
You can use all whole wheat white flour, half whole wheat/half all-purpose, or 1:1 gluten-free all-purpose flour. You may need to add a touch more liquid if using all whole wheat or the gluten-free flour blend. You'll likely be good to use 1/3 cup all-purpose flour as well. Because the batter is baked in a small skillet, the ratios are somewhat forgiving. GRAIN-FREE PALEO-FRIENDLY SUB: Use this brownie skillet recipe, fold in these crushed cookies, continue with browning marshmallows. recipe adapted from my Whole Wheat Chocolate Chip Skillet Cookie