For the soufflé, preheat the oven to 220C/425F/Gas 7.
Lightly grease four large ramekins with the butter, then dust with half the caster sugar.
Whisk the egg whites in a large clean bowl until soft peaks form when the whisk is removed. Slowly whisk in the remaining caster sugar and continue to whisk until stiff peaks form.
Gently fold in the vanilla custard, lemon zest and juice until just combined.
Spoon the mixture gently into the prepared ramekins and place onto a baking tray.
Bake in the oven for 7-8 minutes then remove from the oven and serve immediately.
Meanwhile, for the spun sugar, melt the sugar in a frying pan over a medium heat and when it turns into a deep golden colour, remove from the heat and use a fork to flick back and forth over the back of a ladle to form a cage. Set aside to cool and harden, then carefully twist the cage off of the ladle and place onto the plate.
Place 300g/11oz of the raspberries and the lemon juice into a food processor and blend to a smooth purée. Taste the mixture and add icing sugar, to taste. Pass the mixture through a fine sieve into a bowl.
Place the soufflé onto the plate. Fill the cage with a few raspberries then drizzle the sauce around.
