1. Prepare the custard by heating milk, sugar, and cornstarch in a saucepan over medium heat.
2. Whisk egg yolks in a separate bowl and slowly temper them with the warm milk mixture.
3. Return the mixture to the saucepan and cook until thickened, stirring continuously.
4. Remove from heat and stir in butter and vanilla extract, then let the custard cool slightly.
5. Briefly dip the biscuits in milk to soften them.
6. Arrange a layer of dipped biscuits in the bottom of a baking dish.
7. Spread a layer of custard evenly over the biscuits.
8. Repeat the layers of biscuits and custard until all ingredients are used, finishing with a custard layer.
9. To make the chocolate ganache, heat heavy cream until warm, then pour it over chopped chocolate.
10. Stir the chocolate and cream mixture until smooth and glossy.
11. Pour the chocolate ganache over the top of the layered custard and biscuits.
12. Smooth the ganache with a spatula and add optional decorations like chocolate shavings or crushed biscuits.
13. Cover the cake and refrigerate for at least several hours or overnight to set.
14. Once chilled, slice and serve the vanilla no-bake biscuit cake.
15. Store any leftovers in the refrigerator for up to several days.