Preheat oven or grill to 350°F. Grease 10" cast iron skillet with coconut oil or butter.
Place dry ingredients in a medium sized bowl and mix in eggs and vanilla. Gradually add melted butter, stirring until well combined. Pour batter into cast iron skillet and top with graham wafer pieces.
Tip: If cooking on the grill, watch your temperature gauge to make sure it doesn't get too high. Avoid peeking or the heat will escape. I find it works best to have only the front and back heat sources on while baking on the grill.
Remove brownies from the oven or grill. Turn heat up to 400°F.