Preheat your oven to 350°F (175°C) and prepare a baking dish by greasing it or lining it with parchment paper.

Combine graham cracker crumbs, flour, brown sugar, and salt in a bowl. Incorporate cubed butter until the mixture resembles coarse crumbs. Press into the bottom of the dish.

In a saucepan, combine evaporated milk, heavy cream, granulated sugar, corn syrup, and butter. Heat until smooth and simmering, then stir in unsweetened chocolate.

Bloom the gelatin in cold water for 5 minutes. In a saucepan, heat sugar, corn syrup, and water until soft ball stage. Pour over bloomed gelatin and whisk until thick and fluffy.

Spread the marshmallow mixture over the chocolate layer. Chill in the fridge for at least 1-2 hours until set.

Slice the chilled bars into squares using a sharp knife, cleaning the knife between cuts for neat edges.