I am planning to use PPB recipe for chocolate mousse as a cake filling. To make it hold it’s shape, I want to add powdered gelatineI. Any suggestions how much to use and at what stage to incorporate it?

Perhaps add it directly to the whipped cream or to the egg yolk batter while adding the sugar syrup mixture?

I don’t have my book in front of me, and I don’t know exactly how the recipe goes…but I would incorporate the gelatin in the same manner that is used for stablizing whipped cream. You need to bloom it first in some room temp water for a few minutes, then heat it till it melts, and then add it to one of the components of the mousse that will be beaten (probably the cream). As to the amount?  you are on your own for this one.