Put the sugar in a large heavy-based saucepan with 2 tablespoons (30ml) of cold water. Heat gently, stirring occasionally, until the sugar dissolves, then simmer for 10–12 minutes until caramelised and a rich golden colour.
With the pan over a low heat, gradually and carefully (as the mixture will froth up), pour the evaporated milk into the pan and stir until the caramel dissolves into the milk. (The caramel will harden when the milk is first poured in, but will dissolve again on heating.) Remove from the heat to cool.
Meanwhile, melt the chocolate in a heatproof bowl set over a saucepan of simmering water, without letting the bowl touch the water. Allow to cool for 5 minutes, then beat in one egg yolk at a time, then the caramel mixture.
Whisk the egg whites in a clean bowl until soft peaks form, then gently fold them into the chocolate mixture. Divide among four individual dishes. Refrigerate for at least 1 hour.