In a small bowl, whisk together yolks and sugar. In a medium saucepan over medium-low heat, combine cream, milk and salt; pour in yolk-sugar mixture. Stir continuously until the temperature registers 175° to 180° on an instant-read thermometer or it coats the back of a wooden spoon, about 15 minutes. (Do not overcook, as the eggs could scramble.)
Immediately pour hot mixture over chocolate in a bowl. Cover with plastic wrap and let stand 3 minutes. Uncover, add vanilla and stir to completely combine.
Pour into 8 4-ounce ramekins and let cool. When room temperature, cover each with plastic wrap, and refrigerate for at least 6 hours or overnight.
Before serving, sprinkle with a pinch of sea salt flakes.