Step 1: Make the Custard Filling
In a medium saucepan, crack:
- Egg and whisk until fully blended
- Granulated sugar
- Vanilla sugar or extract
- Cornstarch
Whisk hard for 30 seconds until smooth and slightly thickened.
Gradually add:
- Sour cream , one-third at a time, whisking well after each addition.
Step 2: Cook Over Medium Heat
Place the saucepan over medium heat , stirring constantly with a heatproof spatula .
Cook for 6–7 minutes , scraping the bottom and sides until the mixture becomes thick and glossy , like pudding.
Tip: Don’t let it boil—it may curdle or separate.
Remove from heat and stir in:
- Butter cubes , one by one, until fully melted and incorporated
Let cool slightly before spreading.
Transfer to a shallow bowl and cover with plastic wrap directly on the surface.
Cool to room temperature—about 15 minutes .
Step 3: Roll Out the Puff Pastry
Preheat oven to 375°F (190°C) (convection: 350°F / 180°C) .
Line a baking sheet with parchment paper.
On a lightly floured surface, unroll the puff pastry into a rectangle roughly 12 x 16 inches (30 x 40 cm) and about ⅛ inch (3 mm) thick.
Work quickly to keep the pastry cold and firm.
