Butter a 15-20cm tin and line it with baking parchment.
In a large saucepan, gently melt the butter, sugar, syrup, and cocoa over low heat until smooth.
Remove from heat and stir in the crushed digestive biscuits, dried fruits, and chopped nuts until well combined.
In a separate heatproof bowl, melt the milk and dark chocolates either in the microwave or over a pan of simmering water.
Add half of the melted chocolate mixture to the saucepan and mix well.
Pour the combined mixture into the prepared tin, pressing down firmly and smoothing the top.
Spread the remaining melted chocolate over the top of the mixture in the tin.
Optionally, sprinkle a pinch of sea salt over the chocolate layer.
Place in the refrigerator for at least 2 hours.
Once set, remove from the fridge, dust with icing sugar and cut into squares.