Preheat the oven to 220C/425F/Gas 7.
Whisk the egg yolks in a bowl. Add the custard and lemon curd and beat to combine.
Meanwhile, in a separate clean bowl, whisk the egg whites to form soft peaks.
Add the sugar and whisk to form stiff, glossy peaks.
Fold the egg whites into the lemon mixture to combine.
Grease two ramekins with butter and dust with the caster sugar.
Pour the soufflé mixture into the ramekins and bake in the oven for 10-12 minutes, or until well risen and golden.
Serve.