Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper. In a bowl, whisk together the all-purpose flour, baking soda, and fine sea salt. In a separate bowl, melt bittersweet chocolate and unsalted butter together until smooth. Then, whisk in granulated sugar, followed by the egg and vanilla extract. Gradually combine the wet ingredients with the dry mixture, spreading it evenly in your pan. Bake for 13-15 minutes, until set but slightly soft in the center.

Once the brownie layer has cooled, combine chewy caramels and evaporated milk in a bowl. Microwave in 30-second intervals until smooth. Mix in fine sea salt and crispy rice cereal until well combined. Pour this over the cooled brownie layer, spreading it evenly, and freeze for about 30 minutes.

For the nougat layer, melt unsalted butter, granulated sugar, and evaporated milk over medium heat. Bring to a boil for 5 minutes until thickened. Remove from heat and whisk in vanilla extract, creamy peanut butter, and marshmallow fluff until mixed. Pour this over the caramel layer and smooth it out. Freeze for another 30 minutes.

In a bowl, combine bittersweet chocolate, espresso powder, unsalted butter, and heavy cream. Heat in 30-second bursts until melted and smooth. Pour the ganache over the nougat layer, smoothing it with a spatula. Chill for at least 2 hours.

Lift the treat out using the parchment paper, place it on a cutting board, and slice into squares. Sprinkle flaky sea salt over the top before serving.