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

In a food processor, pulse graham crackers, granulated sugar, and fine sea salt until crumbly. Drizzle in melted butter and pulse until the mixture holds together. Press into the bottom of the lined pan and bake for 10 minutes.

Combine semisweet chocolate chips and sweetened condensed milk in a saucepan over medium-low heat, stirring until melted and smooth. Pour over the cooled crust and chill in the refrigerator.

In a mixing bowl, beat egg whites and cream of tartar until soft peaks form, about 3 to 5 minutes.

Cook light corn syrup, granulated sugar, and water in a saucepan over medium heat until it reaches 240°F (115°C) on a candy thermometer. Drizzle syrup into whipped egg whites while mixing.

Continue to beat until thick and glossy, about 6 to 8 minutes. Spread marshmallow topping over the chilled chocolate layer.

Toast the marshmallow topping with a kitchen torch or by broiling in the oven until golden brown and bubbly.

Allow to cool for at least 1 hour, then lift out and cut into bars with a sharp knife dipped in hot water.