In 3-4 cups water, soak the gelatin until it swells, then melt it in a water bath. Leave it aside to cool.
Beat the yolks in 1/2 the sugar and slowly add the flour, vanilla and gelatin. Stir until you get a smooth mixture and pour it into the boiling milk containing the remaining sugar.
Boil until the cream is sufficiently thick and divide it into 3 equal parts. Leave the 1st part as is. Pour the cocoa in the 2nd and stir to obtain a smooth mixture. To the 3rd, add the walnuts and stir again to a smooth mass.
Grind the biscuits and pour them into the form. One by one, pour the 1st part, then the 2nd and the 3rd on top.
Put the Charlotte Cake in the fridge for a few hours to gel well.