1. Heat the Liquids: In a saucepan, combine the milk, water, sugar, salt, and butter. Heat over medium heat until the butter melts and the mixture becomes hot, but do not boil.
2. Add the Flour: Remove from heat and add the flour all at once, stirring vigorously with a spatula until the mixture forms a smooth dough that pulls away from the sides of the pan.
3. Cool the Dough: Transfer the dough to a bowl and allow it to cool for a few minutes.
4. Incorporate the Eggs: Add the eggs one at a time, mixing well after each addition, until the dough is smooth and shiny.
5. Spread the Dough: Preheat the oven to 180°C (350°F). Line a baking sheet (38 x 27 cm) with parchment paper. Spread the dough evenly over the sheet to create a thin layer.
6. Bake: Bake the dough for 30 minutes until golden and puffed. Remove from the oven and let cool.

Prepare the Vanilla Custard Cream:

1. Mix the Eggs and Sugar: In a saucepan, whisk together the eggs, sugar, vanilla sugar, and cornstarch. Add 1/3 of the milk and whisk until smooth.
2. Cook the Custard: Gradually add the remaining milk, whisking continuously. Cook the mixture over medium heat, stirring constantly until it thickens and starts to boil.
3. Add the Butter: Once thickened, remove from heat and stir in the butter until fully incorporated. Set aside to cool slightly.

Assemble the Roulade:

1. Spread the Cream: Once the choux pastry has cooled, transfer it to a work surface lined with parchment paper. Spread the custard evenly over the entire surface of the pastry, leaving a 1 cm border around the edges.
2. Roll the Pastry: Gently roll the pastry into a log, using the parchment paper to help guide the roll. Wrap the roulade tightly in parchment paper and refrigerate for at least 5 hours or overnight.
3. Serve: Before serving, dust the roulade with powdered sugar. Slice and serve.

Serving Suggestions:

• Serve with a dollop of whipped cream for added richness.
• Garnish with fresh berries like raspberries or strawberries.
• Drizzle with chocolate sauce for a decadent finish.
• Serve chilled with a cup of hot coffee or tea.
• Add a sprinkle of chopped nuts, such as almonds or pistachios, for added texture.