Add a couple inches of water to a saucepan and bring to a boil on the stovetop, this will serve as your bain marie, water bath. Once boiling, lower heat to simmer.

Add egg yolks, whole egg, sugar, and water to the metal bowl of an electric mixer and whisk by hand to combine. Using a hotpad to protect your hand, hold this bowl over the saucepan of water, whisking until mixture hits 185º F.

Remove from heat, dry outside of the bowl, place in the stand mixer with whisk attachment, and whisk on high for 15 minutes, until mixture is cool and billowy. Transfer mixture to a bowl and set aside as you'll use the whisk and mixing bowl to whip cream next.

Pour 1½ cups heavy cream into mixer and whip until firm peaks. Place in refrigerator while you prepare the chocolate.

Place chopped chocolate and ½ cup heavy cream in a small bowl. Set this in a shallow pan of barely simmering water until melted. Remove from water and allow to cool slightly.

Fold the chocolate into the pâte à bombe (egg yolks) then fold in the chilled whipped cream. Transfer to a piping bag fitted with a star tip (Ateco 826 or what you like) and pipe spirals atop baked Overnight Hazelnut Dacquoise layers, ending with chocolate mousse on top.

Sprinkle with additional cocoa powder and/or confectioners sugar. Crush extra bits of baked dacquoise, and sprinkle on top.

Chill for at least 3 hours or overnight.