In a medium bowl, whisk the egg yolks with the sugar until blended. Gradually whisk in the hot cream. Pour the mixture in a clean saucepan and cook over moderate heat, stirring constantly with a wqooden spoon, until it reaches 160°F, is slightly thickened and just coats the back of a spoon, 6 to 8 minutes. Don't let the custard boil, or it will curdle.