In a pan, bring the milk to a boil.
In the meantime, beat the egg yolks with the cornstarch and the cocoa powder until smooth.
Carefully pour the warm milk on the egg mixture, making sure to mix continuously with the hand whisker.
Bring this mixture to the heat again, whisking during the process. Let it thicken and start bubbling.
Remove from the heat and transfer the cream to a bowl. Cover it with cling film and place it in the freezer.
In the mixer, beat the egg whites with a pinch of salt until stiff peaks form.
One spoonful at a time, add the powdered sugar with the mixer still on.
In the microwave or over a double boiler, melt the chocolate. Lower the speed setting of your mixer and slowly pour in the melted chocolate.
In a separate bowl, beat the heavy cream until stiff peaks form.
Remove the cream from the freezer, which should now be firm and cold.
Mix a third of the meringue with the cream. Using a spatula, carefully fold in the second third, and then the last third.
Finally, carefully fold in the whipped cream, just like in the previous step.
Cover the mousse with cling film and place it in the fridge for, at least, 5 hours.
Split the mousse between the cups previously prepared with the caramel. Smoothen with a spatula.
Decore with chopped walnuts and a dash of sea salt. Serve immediately.