To make the caramel swirl, place the sugar in a very clean heavy-based saucepan and pour over the warm water. Brush the sides of the saucepan with a pastry brush dipped in water to rinse away any sugar crystals that may have stuck to the sides. Heat over a low heat, stirring occasionally with a wooden spoon, until all the sugar has dissolved. Turn up the heat and boil until the sugar turns to golden caramel. Dip the base of the pan momentarily in cold water to halt the cooking process, then leave the caramel to stand for a few minutes to allow the caramel to become viscous. Pour a thin line of caramel in a swirl onto oiled baking paper and leave to cool.
For the lemon posset, mix together the mascarpone, icing sugar and lemon curd. Fold in the whipped cream and milk. Put the crushed shortbread biscuits into the bottom of a tall glass, put 2 tablespoons of the raspberries onto the biscuit and then top with a layer of the posset mixture. Top with the remaining raspberries.
To serve, stick the caramel swirl into the glass and garnish with a sprig of mint.