In the top of a double boiler, melt the butter over simmering water. Add the sugar and whisk to combine, whisking for 1 minute. Add the cider and whisk until the sugar is dissolved, 2 minutes. Remove from the heat and add the egg yolks, 1 at a time, whisking constantly. Return to the heat and continue whisking until the sauce is pale and slightly thickened, 5 minutes.
Remove from the heat.
Spoon the sauce over the pudding and serve immediately. (Note: The sauce will thicken and set up as it cools. If you desire a thicker consistency, serve the sauce slightly cooled.)