Make sure that the sauce and brownies are at room temperature before starting with the mousse.
Beat the cream until you reach chantilly stage. It should look soft and cloud-like. Keep the cream in the fridge where it will get extra consistency while we go on with the rest of the recipe.
Soften the lucuma pulp with the back of a fork before starting. If you fell it's a bit fibrous you can pass it through a colander afterwards.
Place the sugar and water in a saucepan and make it boil over medium-low heat. You want to see it in a real rolling boil!
When the syrup is about to boil start beating the egg yolks at high speed. Add the syrup down the side of the bowl so it doesn't touch the whisk. Keep beating on high speed until the mix completely cools down. It should look lovely and foamy. This technique of syrup-cooked egg yolks is called a "pâté à bombe".
Add ⅓ of the egg yolk mix to the lucuma pulp and mix with a spatula. Then add this to the rest of the yolks and use a spatula and folding motions to mix it in.
Add the cream and also fold it together.