Fit a bowl over a saucepan of barely simmering water, and put the egg yolks and sugar into the bowl.
Using an electric whisk, start whisking the yolks and sugar over the simmering water. The mixture should become pale and thick like super-aerated double cream, and by the end it should have at least tripled in volume and be exuberantly frothy. This could take up to 15 minutes.
Continue whisking as you slowly pour in the Prosecco; you should then have a soft, billowing mass. Remove the bowl from its pan of simmering water, and place on a cold surface or stand it in a pan of cold water, and whisk until cool. Then whisk the whipping cream and fold into the egg mixture. Pour into 2 glasses, garnish with chocolate shavings, if using, and serve.