Whisk the yolks and sugar in a large glass bowl to blend. Set the bowl over a saucepan of simmering water (do not allow the bottom of the bowl to touch the water). Whisk until the yolk mixture is very thick and fluffy, about 3 minutes. Whisk in the hot espresso. Pour the espresso mixture into mugs and serve immediately.
Recipe courtesy of Giada De Laurentiis