Line your square pan with parchment paper, leaving some overhang for easy lifting later.
Place the chopped dark chocolate in a microwave-safe bowl, then melt it in short 30-second bursts, stirring well after each, until smooth and glossy. This should take about 1-2 minutes total. The chocolate will smell rich and start to thicken as it melts.
In a large bowl, combine the crispy rice cereal, chopped nuts (if using), and sea salt. Toss gently to distribute evenly, creating a crunchy, textured mixture.
Pour the melted chocolate over the dry mixture. Use a spatula to fold everything together carefully, ensuring all the cereal is coated with chocolate and sticky enough to hold together. The mixture will become shiny and slightly thickened.
Transfer the chocolate-coated mixture into your prepared pan. Press it down firmly with the back of the spatula or a piece of parchment, creating an even, compact layer. This helps the bars hold their shape once set.
In a small saucepan or microwave, melt the butter and vanilla extract together until just warm and combined. Drizzle this over the pressed mixture to add extra richness.
Place the pan in the fridge and chill for at least 2 hours, or until the chocolate is firm and the bars are set. For quicker setting, you can freeze for about 30 minutes, but don’t forget to time it.
Once firm, lift the entire block out of the pan using the parchment overhang. Use a sharp knife to cut into even rectangles or squares, wiping the blade clean between cuts for neat edges.
If desired, melt a small amount of chocolate and drizzle over the top of the bars for a decorative finish. Let the drizzle set for about 15-20 minutes in the fridge.
Serve the bars immediately or store them in an airtight container in the fridge for up to a week. For a softer texture, let them sit at room temperature for 10 minutes before enjoying.
