Preheat the oven to gas 6, 200°C, fan 180°C. Grease 4 miniature loaf pans (or pudding moulds) with butter. Add ½tsp caster sugar to each and tip around to coat. Place on a baking tray.
Melt the chocolate and butter together in a bowl set over a pan of gently simmering water.
Beat the eggs and extra yolk for 2-3 minutes in a separate bowl until light and thickened. Sift in the icing sugar and whisk until combined then fold in the melted chocolate mixture.
Divide the mixture between the prepared moulds. Push 1 square of the caramel chocolate into the top of each mould.
Bake in the oven for 12 minutes until just firm on top, but still runny in the centre.
Leave to rest for 1 minute then loosen, turn out and dust with icing sugar. Serve with raspberries.
Cook's tip: You can prepare these ahead and keep chilled, covered, in the fridge until ready to cook. They will need about 2 minutes extra cooking time.