Put the egg yolks and sugar in a large bowl set over a saucepan of simmering water. Use a hand-held electric whisk or a balloon whisk to beat the yolks and sugar for 2–3 minutes until they are thick and pale.
Whisk in the white wine or sherry, little by little, and continue whisking for about 5 minutes until the mixture is light, thick and foaming.
Warm the blueberries in a small saucepan with 1 tablespoon of water and spoon them into the bases of 6 small glasses. Whisk the mint into the foaming wine mixture and pour it over the blueberries. Stand the glasses on small plates or on a tray and arrange a few extra berries around them. Top with a little chopped mint and serve immediately.