Vanilla Bean Cream Filling

Combine the egg, 2 egg yolks, sugar, cornstarch, and salt in a mixing bowl. Thoroughly blend the ingredients until they merge into a smooth, pale yellow mixture that has thickened slightly, taking around 5 to 8 minutes.

Heat the milk in a saucepan. Open the vanilla bean lengthwise and scrape out its seeds using a paring knife, releasing their fragrance. Add these fragrant seeds to the milk. Warm the milk over medium-high heat, stirring occasionally, until it starts to gently boil and a few tiny bubbles appear on the surface. Take the milk off the heat.

Gradually whisk in the hot milk into the egg mixture, starting with a small amount and then incorporating the rest. Pour the combined mixture back into the saucepan.

Place the saucepan over medium-low heat and continue to whisk until the mixture starts to thicken, taking around 3 to 5 minutes. Remove it from heat and let it cool for 5 minutes.

Add chunks of butter to the mixture and whisk until they melt into the warm, creamy liquid. Pass the mixture through a fine mesh strainer to remove any remaining egg chunks or vanilla seeds. Cover the surface with plastic wrap to prevent discoloration and refrigerate until it has chilled thoroughly, taking around 2 to 3 hours.