Preheat your oven to 350°F (175°C). In a mixing bowl, whisk together the all-purpose flour, baking soda, and fine sea salt. Melt the bittersweet chocolate and unsalted butter together over low heat until smooth, then remove from heat and mix in the granulated sugar, egg, and pure vanilla extract. Combine this mixture with the dry ingredients, then pour it into a lined 8x8-inch pan, spreading it evenly. Bake for 13–15 minutes or until just set, then let it cool completely.

In a microwave-safe bowl, melt the chewy caramels combined with evaporated milk and a splash of water in 30-second intervals, stirring until smooth. Once melted, gently fold in the crispy rice cereal and a pinch of flaky sea salt. Spread this mixture over the cooled brownie layer, pressing down lightly, then place it in the freezer for about 30 minutes to firm up.

Combine butter, granulated sugar, and evaporated milk over medium heat. Stir constantly until the mixture reaches a gentle boil and sugar is dissolved. Remove from heat and mix in the peanut butter and marshmallow fluff until entirely smooth. Pour this nougat mixture over the salted caramel crunch layer, ensuring an even spread, then return the pan to the freezer for another 30 minutes.

Melt together bittersweet chocolate, espresso powder (if using), butter, and heavy cream in a small saucepan over low heat, stirring continuously until fully melted and smooth. Pour the ganache evenly over the top, smoothing it out with a spatula. Chill in the refrigerator for about 2 hours or until the ganache has set.

Carefully remove the dessert from the pan using the parchment paper edges. Use a sharp knife to cut into squares, wiping the knife clean after each cut for neat edges. Sprinkle flaky salt on top for that extra touch, then enjoy!