Photograph: Martin Poole
For the base, melt the butter with the cocoa in a heavy-based pan, stirring until smooth. Remove from the heat and stir in the milk chocolate until melted. Mix in the crushed biscuits. Press into the base of a nonstick 18-20cm square cake tin lined with nonstick baking paper. Set aside.
For the caramel, heat the butter and sugar in the cleaned pan, stirring until melted and combined. Add the condensed milk, bring to the boil, then cook over a gentle heat for 6-7 minutes, stirring until thickened. Pour over the base, cool, then chill for 1 hour.
Melt the dark and milk chocolate in a bowl; melt the white chocolate separately. Spread the dark/milk mixture over the caramel, then drop teaspoonfuls of the white chocolate over the surface. Gently swirl it in. Chill for 4-5 hours. Cut into 16 and serve (the chocolate layer will cut better at room temperature).
Get ahead: the squares will keep for up to a week in an airtight container.
Now try these fabulous variations:
For the base, use oaty biscuits instead of digestives. Add 1 tbsp instant coffee to the cocoa/butter mixture and stir in 25g toasted chopped pecans. For the caramel, dissolve 1 tbsp instant coffee in 1 tsp boiling water then mix into the caramel. For the topping, use 300g milk chocolate. Scatter with 75g toasted, chopped pecans.
White chocolate, cranberry and pistachio
For the base, add 50g golden syrup to the butter; omit the cocoa and milk chocolate. For the topping, use 300g white chocolate. Scatter over 25g roughly chopped dried cranberries and 15g chopped pistachios.