Combine egg yolks, condensed milk, and vanilla extract in a mug or glass. Place the mug in a bowl of hot water. This tip will help reduce the eggy smell and pasteurize the eggs.
Use a hand mixer with a whisk to beat the egg yolk until it turns pale yellow and creamy.
To check readiness, drop some egg cream into water; if it floats, it's ready.
Pour the egg cream onto the coffee, sprinkle cocoa powder on top.
Enjoy the hot coffee; the eggy smell may develop as it cools.