Ingredients :

• The egg yolk — This is what does all the work. Choose a very fresh farm egg, with a bright orange yolk—not the pale yellow of battery eggs. The more colored it is, the more beautiful and flavorful the foam will be. One is enough per cup.
• The coffee — Here, no question of filtered coffee that is too light. You need a short espresso, or a strong coffee brewed in a moka pot. The coffee must stand up to the sweetness of the foam. An Arabica that is too light would be overwhelmed—opt for a blend with Robusta if you can.
• Honey or sugar — Vietnamese people use classic sugar. But acacia honey brings an additional roundness that pairs well with coffee. A level teaspoon, no more—the foam is already naturally sweet once emulsified.
• Sweetened condensed milk (optional) — In the original Hanoi version, this is a key element. It further thickens the foam and softens the whole. If you want to stay in the authentic cà phê trứng spirit, add a tablespoon. Otherwise, skip it—it’s not essential.