Line an 8x8 inch baking pan with parchment paper, ensuring overhang on two sides for easy removal.

In a large mixing bowl, combine the toasted almonds, walnuts, peanuts, and puffed rice cereal.

In a small saucepan, combine honey, brown rice syrup, and fine sea salt over medium heat.

Bring the syrup mixture to a boil and cook until it reaches 260 degrees Fahrenheit (127 degrees Celsius) on a candy thermometer.

Remove the saucepan from heat and stir in the vanilla extract.

Immediately pour the hot syrup over the nut mixture and toss rapidly with a spatula until all components are evenly coated.

Transfer the mixture to the prepared baking pan and press down very firmly using a greased spatula or a second piece of parchment paper to create a level surface.

Allow the mixture to cool and harden at room temperature for approximately 1 hour.

Melt the dark chocolate in a microwave-safe bowl in 30-second intervals, stirring between each, until completely smooth.

Drizzle the melted chocolate over the cooled nut slab and immediately sprinkle with flaky sea salt.

Place the pan in the refrigerator for 15 minutes to set the chocolate.

Lift the slab out of the pan using the parchment overhang and slice into 12 rectangular bars using a sharp chef's knife.