Prepare the Cake Batter

1. Mix Ingredients:
   • In a saucepan, combine water, milk, salt, sugar, and butter. Heat over medium until the butter melts completely.
2. Add Flour:
   • Add flour to the mixture and stir vigorously until smooth, with no lumps. Continue cooking for 2-3 minutes, stirring constantly, until the dough pulls away from the sides of the pan.
3. Cool and Add Eggs:
   • Transfer the mixture to a bowl and let it cool slightly.
   • Beat in the eggs one at a time, mixing well after each addition until the dough is smooth and pulls away from the spatula.

Bake the Cake

4. Preheat Oven:
   • Preheat your oven to 200°C (395°F).
5. Bake:
   • Spread the batter evenly on a baking tray lined with parchment paper, using a spatula to smooth the surface.
   • Bake for 30 minutes or until golden brown. Remove from the oven and let it cool completely.

Prepare the Custard Filling

6. Mix Custard Base:
   • In a mixing bowl, whisk together the eggs, sugar, and cornstarch until smooth.
7. Heat Milk:
   • In a saucepan, heat milk and vanilla until just beginning to boil.
8. Combine and Cook:
   • Gradually add the hot milk to the egg mixture, whisking constantly to prevent curdling.
   • Pour the mixture back into the saucepan and cook over medium heat, stirring constantly, until thickened.
9. Add Butter:
   • Remove from heat and stir in the butter until fully melted and smooth. Allow the custard to cool completely.

Assemble the Cake Roll

10. Spread Custard:
    • Once the cake and custard are cooled, spread the custard filling evenly over the cake.
11. Roll the Cake:
    • Carefully roll up the cake, using the parchment paper to guide it into a tight roll.
12. Chill:
    • Wrap the rolled cake in plastic wrap and refrigerate for at least 1 hour to set.

Serve

13. Finish and Slice:
    • Before serving, dust the cake roll with powdered