Whisk egg yolks with sugar until pale and thick, then whisk in cornflower.
Add cocoa powder to milk and bring to a boil before switching off heat.
Slowly pour milk into egg mixture while whisking vigorously to avoid scrambling.
Return mixture to a clean pot over medium heat and whisk continuously.
Cook until the cream thickens and releases a bubble, then remove from heat.
Pour into a shallow bowl, cover directly with cling film, and refrigerate for one hour.
Whisk half the egg whites with icing sugar, lemon juice, and salt until white.
Add remaining egg whites and whisk until the mixture forms stiff peaks.
Melt dark chocolate in a heatproof bowl over simmering water or in the microwave.
Whip the cream until soft peaks form.
Beat the chilled pastry cream and stir in melted chocolate followed by meringue and whipped cream.
Brush ramekins with butter and coat sides with cocoa nibs.
Divide mousse into glasses and chill for at least one hour before serving with cocoa nibs.