Line an 8×8 inch (20×20 cm) square pan with parchment paper, leaving some overhang so you can easily lift the bars out later.
For extra crunch, spread the puffed buckwheat on a baking sheet and toast it at 160°C (320°F) for 8-10 minutes, then let it cool completely. This step is optional but adds more crunch.
Melt the dark chocolate in a heatproof bowl using a double boiler, stirring between each interval until smooth.
Stir the hazelnut butter, honey, vanilla, and salt into the melted chocolate until the mixture is smooth and fully combined.
Add the puffed buckwheat and gently fold it into the chocolate mixture until evenly coated.
Transfer the mixture to the prepared pan and press it down firmly with a spatula or spoon so the surface is even and compact.
Refrigerate for at least 2 hours, or until the bars are completely firm.
Lift the slab out using the parchment paper, sprinkle with flaky sea salt if desired, and cut into 16-18 bars or squares.
