How to Make It
In a small saucepan, combine the sugar and water and bring to a boil over moderately high heat. Cook, without stirring, until the syrup reaches 250° on a candy thermometer, 4 to 6 minutes.
Meanwhile, in the bowl of a stand mixer fitted with the whisk, beat the egg whites at medium-high speed until soft peaks form. With the mixer on, gradually pour in the hot syrup in a steady stream and beat at high speed until the whites are stiff, 2 to 3 minutes. Cover the meringue with plastic wrap and let stand at room temperature.
Put the chocolate in a heatproof bowl. In a small saucepan, heat the milk just to a simmer. Pour the milk over the chocolate and let stand for 1 minute, then stir until smooth and let cool.
In a bowl, beat the cream to soft peaks. Reserve 1/2 cup of the whipped cream for serving.
Scoop half of the meringue into a bowl (reserve the rest for another use). Whisk in the remaining whipped cream.
Warm the chocolate mixture in a bowl set over a pan of simmering water, stirring, until just melted. Pour the chocolate over the meringue and quickly fold it in. Spoon the mousse into glasses, swirl in the reserved whipped cream and serve.