Citrus-Infused Custard Sauce

Bring milk and cream to a simmer with citrus zests in a 2-quart heavy saucepan over medium-low heat. Whisk together eggs, sugar, and salt in a medium bowl, then whisk in hot cream mixture. Return mixture to pan and cook over medium-low heat, stirring constantly with a wooden spoon, until custard is thick enough to coat back of spoon (160 ° to 165 °F on an instant read thermometer), about 2 minutes. Immediately remove pan from heat and strain custard through a fine sieve into a bowl, discarding zests. Serve warm, or at room temperature, or chill to thicken.