Line your square baking dish with parchment paper for easy removal.
Chop the dark chocolate into small, even pieces and melt it in a heatproof bowl over a saucepan of simmering water or in short bursts in the microwave, stirring until smooth and glossy. Let it cool slightly while you prepare the other ingredients.
Toast the mixed nuts in a dry skillet over medium heat until fragrant and golden, about 5 minutes. Transfer them to a plate to cool, then roughly chop them for easier mixing.
In a large mixing bowl, combine the toasted nuts with the butter and honey. Mix well until the nuts are coated and sticky, creating a cohesive mixture.
Pour the melted chocolate over the nut mixture and gently fold together with a spatula until everything is evenly coated. If the mixture feels too thick, warm it for a few seconds to loosen it up.
Transfer the chocolate-nut mixture into your prepared dish. Use the back of a spatula to press firmly and evenly into the corners, smoothing the top for a flat surface.
Sprinkle a pinch of flaky sea salt on top for added flavor contrast, then place the dish in the fridge to set for at least 2 hours, or preferably overnight.
Once fully set, lift the slab out of the dish using the parchment paper and transfer onto a cutting board. Use a sharp knife to slice into 12 neat squares.
Serve immediately or store the slices in an airtight container in the fridge for up to a week. Enjoy the rich, crunchy texture with a glass of milk or tea.
