Prepare your tin by lining it with baking paper. Set aside.
Put the cookies into a food processor and grind until small crumbs. Put into a bowl. Melt the butter in a small saucepan and pour together with the cookie crumbs. Mix until it is a slightly wet mass. Put into your baking tin and press into the bottom with the back of a spoon. Let it set in the fridge for at least 30 minutes.
Place the gelatine leaves in a bowl with cold water.
Place the mon chou in a bowl with the sugar and the vanilla and mix until smooth. Add the double cream (but keep 2 tbsp apart) and mix until you have a soft firm mixture. (like whipped cream but thicker)
Take a very small saucepan and put the 2 tbsp of double cream plus the juice of lemon together and heat gently. It doesn't have to be very hot (and definitely don't let it boil!) but just warm enough to melt the gelatin. Squeeze the water out of the gelatin and add to the warm mix. Stir until dissolved and than add to your creammixture. Also add the zest of the lemon and use a spatula to stir it through.
Place in the tin and smooth the surface with a spatula.