Whisk together milk, flour, cornstarch, sugar, and egg in a saucepan.
Cook over medium heat, stirring constantly until thick and smooth.
Cover with plastic wrap touching the surface and chill for 1 hour.
Add whipped cream powder and vanilla to the chilled mixture.
Beat until smooth and fluffy.
Fold in hazelnut crumbs if desired.
Arrange a layer of biscuits in a serving dish.
Spread cream over the biscuits.
Add chocolate chips or fruit if desired.
Repeat the layers until all ingredients are used.
Heat the liquid cream until it reaches a gentle boil.
Remove from heat and add the chopped chocolate.
Stir until smooth and glossy.
Pour the ganache over the dessert.
Decorate as desired.
Refrigerate overnight before serving.