Vietnamese Egg Coffee

1.Place the egg yolks, condensed milk, and sugar in a heatproof bowl set over a medium, heavy-bottomed saucepan filled with a few inches of simmering water (be sure the bottom of the bowl does not touch the water).
2.Cook over medium heat, using a whisk to stir the mixture and scrape the sides of the bowl occasionally, until the mixture has thickened and is able to coat the back of a spoon, about 5 minutes.
3.Add the vanilla, then whisk the egg yolk custard. Remove the bowl from heat and set on a wire rack. Use a handheld electric mixer to mix in the vanilla on low speed, about 15 seconds. Turn the mixer to medium-high and whisk the custard for 2 to 3 minutes, until light, fluffy, and doubled in volume.
4.Pour the coffee into a heatproof glass. Use a rubber spatula to scrape the egg yolk mixture over the coffee. Garnish with a dusting of ground cinnamon. Serve immediately with a spoon.