Preheat the oven to 150°C (300°F). Line baking trays with parchment paper and lay the puff pastry sheets on top. Allow the sheets to thaw completely.
Prick the pastry surfaces with a fork to prevent excessive puffing. Cover each sheet with another piece of parchment paper and place a second tray on top to weigh them down.
Bake for 20 minutes or until golden and crisp. Cool completely. Using a 9-inch square baking tin, trim each pastry sheet to fit the tin.
Prepare the Vanilla Custard:
In a large heatproof bowl, combine caster sugar and corn starch, whisking to blend.
Add egg yolks and whisk until smooth. Gradually incorporate 1/2 cup of warm milk while whisking to temper the eggs.
Slowly add the remaining milk and vanilla bean paste, whisking consistently.
Transfer the mixture to a saucepan. Cook over medium heat, whisking constantly, until it thickens, approximately 5-6 minutes.
Remove from heat and stir in the butter until fully melted and incorporated.
Assemble the Slice:
Line the baking tin with parchment paper, ensuring it covers the bottom and sides.
Place one trimmed pastry sheet at the bottom of the tin. Pour the custard over the pastry, smoothing it with a spatula for an even layer.
Top with the second pastry sheet. Refrigerate overnight to allow the custard to set firmly.
Slice and Serve:
Carefully lift the dessert from the tin using the parchment paper. Transfer to a cutting board.
Use a sharp serrated knife to slice into portions, wiping the blade clean between cuts. Dust the top with powdered sugar before serving.
Notes
To make this recipe gluten-free:Substitute the puff pastry with a high-quality gluten-free puff pastry. Ensure it is rolled and handled gently as gluten-free doughs are more delicate.Use corn starch or a certified gluten-free thickener in the custard to maintain its creamy texture.