Pour the crushed biscuits into a bowl. Add the melted butter and mix well until all the biscuits are soaked. Pour this mixture into a baking dish (a glass dish or a gratin dish will do) and press down firmly with the back of a spoon to create a compact layer. Place the dish in the refrigerator while you prepare the cream.
2. Make the vanilla cream
Pour the milk into a saucepan. Add the egg yolks, sugar, cornstarch, and vanilla. Whisk well before heating. This is important to avoid lumps.
Place the saucepan over medium heat. Stir constantly with a wooden spoon or whisk. After a few minutes, the cream will thicken. As soon as it becomes smooth, remove from the heat.
Pour the still-warm cream over the biscuit base. Spread it gently with a spatula or the back of a spoon. Let it cool for about ten minutes at room temperature before adding the ganache.
