Make this only after you've assembled the cake. Prepare the gelatin by the instructions provided on the package (I mixed it with water and let it bloom for 5-10 min).
Bring the cream with the scraped vanilla seeds and vanilla beans to a simmer.
Mix the egg yolks with the sugar and slowly pour in half of the cream, stirring constantly. Add this mixture back to the pan with the remaining cream and cook, stirring constantly, until it reaches 82°C. You can check if the cream is done by placing a spoon in the cream and then making a line with your finger across the spoon. If the line stays clean and the cream is slightly thickened, it’s done. Do not overcook, the eggs will scramble! Remove from heat and leave to cool to room temperature, don't let the gelatin set. Remove the vanilla beans, Now you have creme anglaise.
Mix the mascarpone for a few seconds, until creamy. Add the creme anglaise and mix on very low speed until the mixture is combined.
As soon as you make the mascarpone mousse, pour it over the cake, right up to the edges of the pan, or the acetate sheet. Cover with cling film and place in the freezer for a few hours, or overnight. When the cake is frozen, make the mirror glaze.