In a large heat-resistant bowl set over a pan of simmering water, melt the chopped chocolate, stirring occasionally with a silicone spatula.

Meanwhile, wash and dry the bowl of your stand mixer so that it’s spot- lessly clean. Using the whisk attachment, whisk the egg whites and lemon juice on medium speed until they form soft peaks. Increase to high speed and gradually add the sugar, whisking until the egg whites form soft peaks.

Whisk one-third of the egg whites into the warm chocolate, then immedi- ately use a silicone spatula to fold in the remaining egg whites. This step must be done quickly, since you’re adding a cold mass to a hot mass and you don’t want the chocolate to solidify, which would result in grainy mousse. That’s why it’s important that the egg whites are at room temperature. Be careful not to overmix, or the mousse will lose its lightness.

Spoon into six glasses or ramekins, or into one big bowl, and let set in the fridge for 3 to 12 hours before serving.