Line an 8x8-inch or 9x9-inch square baking pan with parchment paper, leaving some overhang on the sides for easy lifting. Press the parchment into the corners; a light spray of cooking spray under the parchment can help it stay in place.

In a medium bowl, combine the graham cracker crumbs, granulated sugar, and 1/4 teaspoon fine sea salt. Pour in the 6 tablespoons melted butter and stir until the crumbs are evenly moistened and resemble damp sand.

Press the crumb mixture firmly and evenly into the bottom of the prepared pan, using the bottom of a measuring cup or your hand. Press well so the crust holds together when sliced. Chill the crust in the refrigerator while you prepare the fudge layer.

Place the chocolate chips, sweetened condensed milk, and 2 tablespoons butter in a large microwave-safe bowl. Microwave on 50% power for 60 seconds, then stir well. Continue microwaving in 20–30 second bursts at 50% power, stirring between each, until the chocolate is mostly melted and smooth with no large lumps.

Once the mixture is smooth, stir in the vanilla extract and 1/4 teaspoon fine sea salt until fully combined. The fudge should be thick, glossy, and spreadable, similar to warm brownie batter.

If preferred, combine the chocolate chips, sweetened condensed milk, and butter in a heavy-bottomed saucepan over very low heat. Stir constantly until melted and smooth, then remove from heat and stir in the vanilla and salt.

Remove the chilled graham cracker crust from the fridge. Pour the warm chocolate fudge mixture over the crust and spread it evenly into the corners with a spatula. Tap the pan gently on the counter to release any air bubbles and level the surface.

While the fudge is still warm, sprinkle the mini marshmallows evenly over the top, pressing them very lightly into the fudge so they adhere but stay fluffy. If desired, sprinkle chopped milk chocolate or chocolate chips and a pinch of extra graham cracker crumbs over the marshmallows.

Using a kitchen torch, gently toast the marshmallows until they are golden brown and lightly charred in spots, rotating the pan as needed for even browning.

Alternatively, preheat your oven broiler. Place the pan on the middle rack and broil for 30–90 seconds, watching constantly so the marshmallows do not burn. Rotate the pan as needed for even toasting, then remove immediately once golden.

Let the pan cool at room temperature for about 30–45 minutes. Once no longer hot, transfer to the refrigerator and chill for at least 2 hours, or up to 3–4 hours, until the fudge layer is fully set and firm enough to slice cleanly.

Use the parchment overhang to lift the entire slab of s'mores fudge from the pan onto a cutting board. Run a sharp knife under hot water, wipe dry, and cut into squares or bars (about 1–1 1/2 inches for party-size pieces). Wipe the knife clean between cuts for neat edges.