Orange surprise pudding

This recipe did not quite work when I made it yesterday. I offer it here because it, or close variations on it, have always worked in the past; and because it tasted fine anyway. I think that the mistake I made was not to blend the batter thoroughly: the creamed sugar and butter were a distinct component of the finished dish, and the custard did not set properly. I used golden caster sugar -- was that the problem? I doubt it.

I first got the recipe, as lemon surprise pudding, from Real Cooking by Nigel Slater. He adapted it from one in Margaret Costa's Four Seasons Cookery Book (from Grub Street -- the link to the food and drink page is not working). It appears, too, in Simon Hopkinson and Lindsay Bareham's Roast Chicken and Other Stories. Serves four.

50 g butter
85 g caster sugar
3 eggs, separated
Zest and juice of one orange (or one lemon)
25 g flour
250 ml milk

Cream the butter and sugar. I do this by hand, working away at the mixture with a spoon or spoons, pressing it to the side of the bowl, until it lightens and looks creamy.

Stir in the egg yolks and the orange zest and juice. The mixture will probably separate. Stir in the flour. Pour in the milk and give this batter a good whisk.

I also whisk egg whites by hand. Instructions for this procedure are often scary, implying that it will not work unless the egg, bowl and whisk meet strict requirements. Yesterday, I thought that I could see traces of yellow in my egg whites; but if there was some yolk there, it did not do any harm. You whisk until the point at which, when you lift the whisk from the white, it creates a peak that does not collapse. Fold the stiffened white into the batter.

Butter an oven dish, place it in a roasting tray, and pour boiling water round it to come half way up the sides. Pour the batter into the dish, and bake at gas mark 3/160 C for 40 to 50 minutes.

The pudding forms a sponge, below which is in theory the surprise: a tangy custard.