Preheat the oven to 350°F. Lightly grease a 10-inch cast-iron skillet.

In a large bowl, beat the softened butter, brown sugar, and granulated sugar until light and creamy, about 2 minutes. Add the egg and vanilla and beat until smooth.

In a separate bowl, whisk together the gluten-free flour, cocoa powder, baking soda, and salt. Add the dry ingredients to the butter mixture and mix just until combined.

Fold in ½ cup of the chocolate chips.

Press the dough evenly into the prepared skillet. Bake for 18 to 22 minutes, or until the edges are set and the center still looks slightly soft.

Remove the skillet from the oven and scatter the mini marshmallows, remaining ¼ cup chocolate chips, and crushed graham crackers over the top.

Return the skillet to the oven for 2 to 4 minutes, or until the marshmallows are puffed and lightly golden. For more dramatic browning, switch the oven to broil for the final 30 to 60 seconds, watching closely.

Immediately sprinkle with Halloween sprinkles and gently press the candy eyes into the warm marshmallows. Let cool for about 5 minutes before serving.

Serve directly from the skillet while the cookie and marshmallows are still warm and gooey.