Puff Pastry with Homemade Custard and Chocolate Drizzle

Ingredients:

1 egg
4 tbsp sugar
2 tbsp cornstarch
8g vanilla sugar
350ml milk
1 tsp butter
Puff pastry sheets
2 tbsp sugar (for sprinkling)
150g powdered sugar
3 tbsp milk (for glaze)
Chocolate for spreading