For the creme patissiere: Whisk the egg yolks with the sugar until pale and thick, and then whisk in the cornstarch.
Add the cocoa powder to the milk and bring to a boil; switch off the heat. Pour the milk in a slow stream onto the egg mixture, whisking vigorously all the time. (Pour slowly to avoid scrambling the eggs.) Return the mixture to a clean pot over a medium heat and whisk continuously. Make sure to scrape the sides and the bottom, otherwise it will burn. The cream will start to thicken. Once it releases a bubble or two, take it off the heat.
Pour into a shallow bowl. Cover with cling film (pat the cling film so it sticks directly onto the cream) and refrigerate for at least an hour before using.
For the meringue: Put half the egg whites into a clean glass or metal bowl. Add the sugar, lemon juice and salt and whisk until white. Add the rest of the egg whites and continue whisking until the meringue forms stiff peaks when the whisk is removed.
Melt the chocolate in a bain marie (a heatproof bowl set over a pot of simmering water) or in the microwave on a low setting. Whip the cream until soft peaks form when the whisk is removed.
To make the mousse, beat the chilled creme patissiere to remove any lumps before stirring in the melted chocolate. Mix in one third of the meringue, and then gently fold in the rest followed by the whipped cream.
To serve, brush 4 to 6 glasses or ramekins with butter. Add some cocoa nibs and roll them around the sides and bottom of the glasses until evenly coated. Divide the mousse among the glasses and chill for at least an hour, but ideally 4 hours. Serve chilled, sprinkled with cocoa nibs. The mousse is best eaten the same day and should not be kept for more than 2 days.
NotesFood Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell. For recipes that call for eggs that are raw or undercooked when the dish is served, use shell eggs that have been treated to destroy Salmonella, by pasteurization or another approved method.