Gather the ingredients.
Combine the egg yolks, condensed milk, and vanilla in the bowl of a stand mixer. Beat with the wire whisker on high for about 10 minutes. The appearance should be similar to a cake batter, not especially frothy.
To test for the right consistency, spoon a small amount of the foam on top of a glass of water. If it floats it has the right consistency. If not, whip it for a few more minutes.
Brew the coffee according to your preferred method. Reserve about half a cup for later use.
Divide the remaining brewed coffee among 4 to 6 cups. If desired, set each cup in a small bowl of hot water to prolong the warmth of the coffee.
Gently spoon some of the whipped egg foam onto the top of each coffee, 1 to 2 tablespoons per cup.
Pour a bit of the reserved coffee through the foam in each cup. Dust with cocoa powder if desired. Serve immediately.
