• Preheat the oven to 350F and grease and line an 8x8 baking pan with parchment paper. The paper is optional but allows you to remove the cake for cleaner frosting and serving.

for the cake

• Whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.
• In another bowl whisk the buttermilk, oil, egg, and vanilla and mix until combined. Stir in the hot coffee.
• Add the dry ingredients to the wet and mix until well combined, but don't overmix. The batter will be thin.
• Pour into the prepared pan and bake for 28–32 minutes, checking at 25 minutes. Let cool completely.

for the ganache

• For the ganache, heat the cream until steaming, then pour over the chocolate, salt, and Nutella. Let sit 2–3 minutes, then whisk until smooth. Spread over the cooled cake.

for the crunch topping

• Add the Nutella, bittersweet chocolate, butter, and pinch of salt to a microwave-safe bowl. Microwave in short 15-second bursts, stirring between each, just until melted and smooth.
• Fold in the lightly crushed cornflakes until they’re completely coated. Stir in the toasted hazelnuts, if using.
• Spoon the mixture over the still soft ganache layer and gently spread it out with an offset spatula or the back of a spoon. Don’t press too hard ~ you want to keep some craggy texture. I dolloped it onto the cake in small mounds and then gently nudged it across the surface.
• Let the topping set before slicing. For the cleanest slices, chill the cake briefly, then let it sit at room temperature for a few minutes before serving.