Whip the heavy cream until soft peaks form. You can use a hand whisk so you don’t overmix it. Cover it with plastic and place it in the fridge to chill.
Place the whole milk in a pot and bring it to a boil. In a separate bowl, mix egg yolks and icing sugar together and pour in the hot milk slowly while mixing it with a whisk. Place it back in the pot over low heat. Keep whisking it until it thickens a little bit. Dip a spoon or spatula in the mixture and run your finger over it (just be careful because it’s hot). If the line doesn’t go back, that means it’s ready, if it does, just continue to cook it a bit more.
In a separate bowl, place dark chocolate chips and put a strainer on top of it. Pour the mixture immediately into the dark chocolate and strain all the egg mixture lumps. Let this mixture cool at room temperature for about 15-20 minutes.
When the chocolate mixture is at room temperature, gently fold in the chilled whipped cream using a spatula. Cover with plastic and let it rest at room temperature. If it’s a bit loose in consistency, just place it in the fridge for about 3- 5 minutes. It’s important that this chocolate mousse filling is of the right consistency (not runny but not too hard).