25 g (1 oz) cocoa powder, sifted, plus extra for dusting
5 egg yolks
50 g (2 oz) caster sugar
150 ml (1/4 pint) Marsala
Divide the biscuits between 6 tall-stemmed glasses. Spoon 1 teaspoon of the Marsala over each.
To make the zabaglione, put the cocoa powder, egg yolks and sugar in a large heatproof bowl set over a saucepan of simmering water. Beat with a hand-held electric whisk until smooth, then gradually whisk in the remaining Marsala. Continue whisking for a further 8–10 minutes or until the mixture is slightly paler in colour, has increased in volume and is foamy.
Carefully spoon the mixture into the glasses. Dust with cocoa powder and serve immediately.