In a medium bowl or jar, beat together the egg yolks, sweetened condensed milk, and vanilla using a hand mixer until thickened and airy with a very pale yellow color, around 5 minutes. Drizzling the mixture should produce ribbons that hold their shape briefly.
Evenly sprinkle the coffee in the filter of a Phin, top with the press to gently compact, and place over a heat safe coffee glass.
Pour in 1 tablespoon (15 milliliters) of the boiling water. Cover the phin and allow to rest for 1 minute.
Add the remaining water, cover, and allow the water to drip through.
Slowly pour the beaten egg yolk and sweetened condensed milk mixture over the back of a spoon just over the hot cup of coffee to create a defined layer.
Serve immediately with a sprinkling of cocoa powder or cinnamon if desired.