Lightly beat in a medium bowl:
Heat in a medium saucepan over medium heat until bubbles form around the edges:
Slowly whisk the hot milk into the egg yolk mixture to warm it. Return this mixture to the saucepan and place over medium-low heat. Stir the sauce gently but constantly with a silicone spatula, sweeping the entire pan bottom and reaching into the corners. As soon as the custard is thickened enough to coat the back of a spoon and reaches 170° to 175°F on an instant-read thermometer, pour it through a sieve into a bowl and stir in:
Serve warm or cold. If serving cold, let the sauce cool thoroughly before covering and refrigerating. Condensation will cause it to thin. The sauce can be covered and refrigerated for up to 3 days. To reheat, scrape the sauce into a double boiler over warm (165°F) water and stir occasionally until warmed through.