Whisk flour, cornstarch and sugar together in a saucepan.
Gradually add milk, then whisk in eggs until smooth.
Cook over medium heat, whisking constantly, until thickened into a smooth custard.
Remove from heat, add vanilla and butter, and stir until fully combined and glossy.
Let the custard cool slightly until warm.
Briefly dip biscuits in milk.
Spread a thin layer of custard in a dish, then layer dipped biscuits and custard, finishing with custard on top.
Place chopped chocolate in a bowl and pour hot heavy cream over it.
Wait one minute, then stir until smooth and glossy.
Pour ganache over the dessert and chill for 1–2 hours before slicing and serving.