Lemon surprise pudding

This is something like a lemon curd souffle, normally advertised as a "light fluffy sponge with a lemon sauce underneath".

120g unsalted butter
200g caster sugar
4 medium eggs, separated
50g plain flour, sifted
finely grated zest of 1 lemon
juice of 2 lemons
400ml milk
icing sugar for dusting
creme fraiche to serve

Heat the oven to 170C fan oven/180C or 350F electric oven/Gas 4. Cream the butter and caster sugar together until pale. Incorporate the egg yolks, then the flour, lemon zest, juice and milk. Whisk the egg whites in a bowl until stiff and fold them into the lemon mixture - if you use a food processor to cream the mixture, then transfer it to a bowl first. Pour the mixture into a buttered 20cm souffle dish. Dust with icing sugar and bake for 30-35 minutes until golden and risen. Serve straight away, with a spoon of creme fraiche.