1. 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 degrees to 180 degrees 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.)
2. 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.
3. 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.
4. Before serving, sprinkle with a pinch of sea salt flakes.