Start by preheating your oven’s broiler to high. While it heats up, grab a medium-sized cast-iron skillet or another broiler-safe pan.

Lay 6 graham cracker squares flat across the bottom of the skillet to form a solid base. Arrange the pieces of chocolate evenly over the crackers, ensuring the surface is mostly covered for even melting.

Nestle the marshmallows on top of the chocolate layer. They can be close together but should have just enough space to expand slightly under the heat.

Carefully place the skillet under the broiler. Toast the marshmallows for 1 to 2 minutes, keeping a close watch—once they start turning golden brown, they can go from perfectly toasted to burnt in seconds.

Using an oven mitt, remove the skillet from the oven as soon as the marshmallows have achieved a golden, caramelized surface. Allow the mixture to cool for a minute or two.

Finish by placing the remaining graham cracker squares on top, pressing them gently into the warm marshmallows. Serve right away while everything is gooey and melty.