For the sauce
Beat the butter with the sugar to a pale cream. Sift in half of the flour, then add one of the eggs, and incorporate. Sift in the remaining flour and beat in, followed by the remaining eggs, to achieve a smooth batter. Add the jam and stir.
Pour into a buttered 1-litre/1 1⁄2pt pudding basin. Cover with a cloth and steam for 1 1⁄2 hours.
To make the sauce, whisk the egg yolks with the sugar in a heatproof bowl set over a pan of simmering water. As they thicken, add the sherry and sieved jam, and cook for a few minutes, stirring constantly.
Serve the sauce hot with the pudding.