Preheat the oven to 170C/150C fan forced. Line 2 large baking trays with baking paper. Use electric beaters to beat the butter and icing sugar in a large bowl until pale and creamy.

Sift in the flour, cocoa and custard powder. Use a flat-bladed knife in a cutting motion to mix until a soft dough forms. Use clean hands to bring the dough together. Wrap in plastic wrap and place in the fridge for 20 minutes to chill.

Roll level tablespoonfuls of the mixture into 12 egg-shaped balls. Place on the prepared trays and press down gently, keeping the egg shape, until about 12mm thick. Bake, swapping trays halfway through cooking, for 18 minutes. Set aside on trays for 5 minutes to cool slightly before transferring to a wire rack to cool completely.

Line a baking tray with baking paper. Place the chocolate melts in a heatproof bowl over a saucepan of simmering water (make sure the bowl doesn’t touch the water). Use a metal spoon to stir until smooth. Working with half the biscuits, dip the wider end of a biscuit into the chocolate. Place on the prepared tray. Sprinkle the gold sprinkles along the edge of the icing across the middle of the biscuit. Set aside to set.

Meanwhile, to make the filling, use electric beaters to beat butter and caramel in a bowl until combined. Gradually add icing sugar, beating constantly until combined. Transfer to a piping bag fitted with a plain nozzle.