Set a strainer over a medium bowl and have it ready by the stove. Heat milk in a small heavy saucepan over medium heat, stirring often to prevent scorching, until steaming and hot but not bubbling. Meanwhile, whisk egg yolks and sugar in a medium bowl.
Gradually whisk the hot milk into the eggs. Pour the mixture back into the pan and place it over medium-low heat. Cook, stirring constantly with a wooden spoon in a figure-eight motion, until the custard thickens enough to coat the back of the spoon, 4 to 6 minutes. The temperature should reach 160°F on an instant-read thermometer (do not let the custard boil). Working quickly, strain the custard into the clean bowl. Whisk in instant espresso (or coffee). Cover and refrigerate until chilled, at least 1 1/2 hours.
Place espresso beans in a sealable plastic bag and crush with a rolling pin or saucepan. Stir into the chilled sauce.