Line an 8- or 9-inch square baking pan with parchment paper, leaving overhang on two sides to create handles. Lightly grease the pan with butter or nonstick spray before adding the parchment so it sticks and the edges don’t stick.

In a medium bowl, combine the graham cracker crumbs, granulated sugar, and 1/4 teaspoon fine sea salt. Pour in the melted butter and stir until the mixture looks like damp sand and holds together when squeezed.

Transfer the crumb mixture to the prepared pan. Use the bottom of a measuring cup or flat glass to press it firmly into an even layer. Chill in the refrigerator for 10–15 minutes to help the crust set while you make the fudge layer.

In a medium heavy-bottomed saucepan over low heat, combine the sweetened condensed milk, semi-sweet chocolate chips, and milk chocolate chips. Stir constantly with a heatproof spatula or wooden spoon until the chocolate is mostly melted. If it starts to bubble, reduce the heat.

Remove the saucepan from the heat and continue stirring until the mixture is completely smooth and glossy. Stir in the vanilla extract and a pinch of salt to balance the sweetness.

Take the chilled crust out of the refrigerator. Pour the warm fudge mixture over the crust and gently spread it into an even layer, reaching all the corners. Tap the pan on the counter a few times to release any air bubbles.

While the fudge is still warm and soft, sprinkle the mini marshmallows evenly over the top. Gently press them down so they adhere to the fudge. Scatter the roughly crushed graham cracker pieces and a few extra chocolate chips over the marshmallows, if using.

For a toasted marshmallow flavor, lightly toast the marshmallow layer with a kitchen torch before chilling. Keep the flame moving and hold it back a bit to avoid burning or catching the marshmallows on fire. Let cool briefly.

Place the pan in the refrigerator and chill for at least 2 hours, or until the fudge is completely set. For best results or for make-ahead, chill for 2–3 hours or overnight. Once the pan is no longer warm, cover it while chilling if storing overnight.

When firm, use the parchment handles to lift the fudge out of the pan onto a cutting board. Use a long, sharp knife to cut into 16 larger squares (4x4) or up to 25 smaller squares (5x5). Wipe the knife with a warm, damp cloth between cuts for the cleanest slices. Let sit at room temperature for 10–15 minutes if very cold before serving.