1. Grease and line a large rimmed baking tray with greaseproof paper.
2. Place the candy canes in a plastic bag and crush with a rolling pin. Set aside.
3. Melt both chocolates in separate heatproof bowls set over half-filled saucepans of simmering water, stirring until smooth.
4. Remove from the heat and stir the peppermint extract into the melted dark chocolate.
5. Pour the dark chocolate onto the lined tray, spread out, and then gently pour the white chocolate on top, swirling it into the dark chocolate with the tip of a knife or a skewer.
6. Scatter the crushed peppermint candy canes, cacao nibs, and some flaky sea salt on top. Chill for 1 hour until set and firm.
7. Remove from the fridge and break into large shards before serving.