Make the Crust

1. Preheat the oven to 350°F. Lightly grease an 8×8-inch metal pan, then line with parchment, leaving overhang on two sides.
   
2. In a food processor or resealable plastic bag with a rolling pin, crush 5 cups (147g) Rice Chex cereal into fine crumbs; it should resemble coarse sand. You should have about 1 1/4 cups fine crumbs.
   
3. Add 6 tablespoons melted unsalted butter, 2 tablespoons light brown sugar, 1 tablespoon powdered sugar, and 1/2 teaspoon kosher salt to the crumbs. Mix until evenly coated – it should look like wet sand and hold together when squeezed. 
   
4. Transfer to the prepared pan and spread into an even layer. Press it firmly into the corners with your fingers, then use the flat bottom of a measuring cup to compact the surface until smooth and very tightly packed, like a solid tile. 
5. Freeze for 15 minutes to firm the butter and help the crust hold its shape.
6. Bake for 15 to 18 minutes, until the edges are slightly darker than the center and the surface feels dry and set when pressed. Cool on a wire rack for at least 20 minutes.

Make the Ganache

1. In a small saucepan over medium heat, melt 2 tablespoons unsalted butter. It will foam, then quiet. Continue cooking, swirling occasionally, until the milk solids turn amber (the color of hazelnuts) and smell nutty, 4 to 5 minutes. Watch closely as it can burn quickly.
2. Remove from heat and carefully add 2 tablespoons heavy cream (it will sputter). Stir, scraping up any browned bits. Add 3 ounces chopped dark chocolate (70%) and let sit for 1 minute, then stir from the center outward until smooth and glossy.
3. Let cool until warm but not hot—about 110°F, or just warm to the touch. If it’s too hot, the Nutella can split.
4. Whisk in the ⅓ cup Nutella, 2 teaspoons white miso, ½ teaspoon vanilla extract, and ½ teaspoon apple cider vinegar until smooth and fully combined, about 1 minute. The mixture should look thick and shiny with no streaks. If it looks split, whisk in 1 to 2 teaspoons warm water, one teaspoon at a time, to bring it back together. Taste and add the additional teaspoon of miso if needed. I usually do. 
5. Pour over the cooled crust and spread evenly.

Make the Hazelnut–Chex Crunch Topping

1. In a medium bowl, gently break up 1 cup Rice Chex into halves and quarters, leaving some larger pieces for texture. Add ¾ cup chopped toasted hazelnuts.
2. Drizzle 1½ ounces melted dark chocolate over the mixture and toss until lightly coated.
3. Add 3 tablespoons powdered sugar and toss until the coating looks sandy and matte, not shiny. If it still looks glossy, add the remaining tablespoon.
4. Scatter evenly over the ganache and press lightly so it adheres. Finish with a generous pinch of flaky sea salt.

Chill and Serve

1. Refrigerate for 1½ to 2 hours, until fully firm.
2. Lift out using the parchment. Run a sharp knife under hot water, wipe dry, and cut into a 4×4 grid (16 bars), wiping the knife between cuts for clean edges.
3. Let sit at room temperature for 15 to 20 minutes before serving for a softer texture, or serve straight from the fridge for a firmer bite.
4. Store bars in an airtight container in the refrigerator for up to 5 days.