Put the egg yolks, condensed milk and vanilla extract in a large mixing bowl. Whisk with electric beaters for at least 5 mins or until pale and foamy.
Put 4 heatproof glasses or ramekins into slightly larger bowls that have been half-filled with boiling water (this will keep the coffee in the glasses hot for longer).
Pour the freshly brewed coffee into the glasses, reserving 2 tbsp, then top with scoops of the egg mixture. Drizzle the remaining coffee on top so that it stains the topping and sinks in. Scatter over a pinch of the grated dark chocolate, if you like. Serve immediately, with spoons.