For the mousse, combine the yolks and sugar in a small bowl and whisk until pale in colour. Place the gelatine sheets in cold water to soften.
Place the 80g of double cream, milk, and praline paste in a medium saucepan, cook over a medium heat, and stir to disperse the praline paste. When the mixture is just below a simmer, remove the pan from the heat.Whisking constantly, pour the mixture onto the yolk mixture, return the mixture to the pan and place over a medium heat.
Stirring constantly bring the mix to a gentle simmer and cook for 1 minute, lowering the heat if necessary to prevent the mixture from curdling, it will be very thick.
Remove from the heat and strain through a fine-mesh sieve into a clean bowl. Squeeze the excess water from the gelatine and add it to the hazelnut custard you’ve just made. Allow the mixture to cool down.
While the custard is cooling, whip the 160g of cream to medium peaks, store in the fridge until the custard is the right temperature.
When the custard had reached an ambient temperature, gently fold in one-third of the cream. When that is evenly mixed, fold in the remaining cream. The mousse is ready to use, take a semi-sphere mould with a cavity that is 3cm diameter and fill the mould. You will have more mousse than you need, I recommend filling at least 9 in case there are any errors, smooth the mousse and remove any excess. Place the mould in the freezer on a tray or board to keep them flat.
The remaining mousse can be stored in the fridge and eaten when set.