Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with foil, leaving an overhang.

Combine graham crackers, granulated sugar, and fine sea salt in a food processor, then add melted butter and pulse until moistened. Press into the prepared pan and bake for 10 minutes.

Melt semisweet chocolate chips and sweetened condensed milk in a saucepan over medium-low heat, stirring until thick and smooth. Pour over the cooled crust and refrigerate.

Beat egg whites and cream of tartar until soft peaks form. Boil sugar, corn syrup, water, and salt until the syrup reaches 240°F, then drizzle into the egg whites while mixing until glossy.

Spread the marshmallow topping over the chocolate layer and toast with a kitchen torch until golden brown.

Refrigerate for at least 1 hour, lift out of the pan, cut into squares, and serve.