Chill 6 x 125 ml (4 fl oz) ramekins while you make the filling. Place the egg yolks, custard powder, lavender sugar and 3 tablespoons of the milk in a heatproof bowl and beat together.
Meanwhile, warm the remaining milk in a saucepan. Gradually whisk into the egg mixture. Return to the pan and cook, whisking continuously, over a medium heat for 3–4 minutes until the mixture has thickened. Leave to cool slightly. Whisk in half of the mascarpone until smooth, then beat in the remaining mascarpone.
Pour the mixture into the ramekins and place in the freezer for 15 minutes.
Place the caster sugar in a small heavy-based saucepan over a low heat and leave until half of it has melted and started to colour, then gently stir it. Keep stirring until it has turned a deep golden caramel colour. Drizzle a little over the top of each ramekin. Leave to cool for a few minutes until the caramel has hardened, then serve.