Step 1In a large bowl, whisk together the egg yolks and sugar until combined and frothy. Whisk in the sherry and the hazelnut liqueur.
Step 2Set the bowl over a large pot of simmering water, making sure the bottom of the bowl does not touch the water. Continue to whisk until the mixture increases in volume and thickens to a thick, foamy cream, easily coating the back of a spoon. The final consistency should be similar to that of a very thick hollandaise.
Step 3Remove from heat and serve immediately. This makes a generous cup of zabaglione.