Line a 9x13-inch baking pan with two crossing sheets of parchment, leaving overhang on all sides to form lifting handles. The parchment sling should sit flat in the bottom of the pan with enough paper extending above the rim to grip later.

Cover the bottom of the pan with graham crackers, breaking sheets neatly along the perforations to fill open spaces. A snug, even layer creates a stable base with no large gaps beneath the filling.

Stir the thawed whipped topping and marshmallow fluff together in a large bowl until fully blended. The mixture should look smooth, pale, and airy, with no streaks of fluff remaining.

Heat the milk chocolate chips in a microwave-safe bowl at 50 percent power for 90 seconds, then stir. Continue in 30-second bursts at 50 percent power, stirring after each, until the chocolate is glossy and smooth.

Spread half of the melted chocolate over the graham cracker layer and scatter the flaky sea salt evenly across it. Use a gentle hand so the crackers stay in place and the chocolate reaches the edges.

Spread all of the marshmallow mixture over the chocolate, then add the remaining chocolate and finish with a second layer of graham crackers. Press the top crackers down lightly until they settle with a soft crackle, without squeezing filling out the sides.

Cover the pan and place it in the freezer for at least 4 hours. The center should feel firm when pressed gently through the top layer before you lift the parchment handles.

Lift the chilled slab from the pan with the parchment overhang and cut it into squares for serving. Clean edges and a firm center show that the layers have set properly.