1. Make the cake layer by first blending the biscuits or crumbling them to a fine sandy texture using a rolling pin - for the latter, put them in a clean bag (not too fine, so it doesn't tear) and roll a rolling pin on top while applying pressure, until you obtain the desired result.
2. Mix the biscuit mixture with the melted butter and stir very well. Pour into a 9 3/4″ (25 cm) form with removable sides and press down with the underside of the spoon, while evening it out at the same time. Put thus prepared cake layer in the fridge to harden while you get to work on the mixture.
3. Beat the confectionery cream while adding the sugar while beating nonstop. Gradually add in the cream cheese, vanilla and finely grated zest of 2 lemons (yellow part only). Squeeze out the juice and heat it, without allowing it to boil. Dissolve the gelatin, hydrated ahead of time, in it and pour the gelatin lemon mixture over everything else, stirring well.
4. Divide the mixture into 2 equal parts, pour the blueberry jam in 1 of them. Pour the 2 mixtures over the hardened layer in at the same time so that they intermix beautifully.