Preheat the oven to 350 degrees F. Line an 8-inch square baking pan with aluminum foil and grease with nonstick cooking spray.

Place the graham crackers in a food processor and pulse until they are crushed. Add the butter, sugar and salt and pulse until the mixture resembles wet sand. Press the crushed graham crackers into the bottom of the prepared sheet pan. Bake for 10-12 minutes until toasted and golden brown. Allow the crust to cool for 15 minutes.

While the crust is baking, melt the chocolate. Bring 1 inch of water to a low simmer in a medium saucepan. Place the chocolate in a heatproof bowl over the saucepan without the water touching the bottom of the bowl. Stir frequently until the mixture is fully melted. (Alternatively, melt the chocolate in the microwave in 30-second blasts stirring to distribute the heat.)

Spread the melted chocolate over the crust and then top with marshmallows pressing them into the chocolate.

Preheat the broiler. Toast the marshmallows until golden brown, about 1-2 minutes. Let the bars cool before cutting into 16 squares.