Begin by gathering all your ingredients. Separate the egg yolks and measure out the whole milk, passion fruit puree, fine sugar, and cornstarch.

In a medium saucepan, combine the whole milk and passion fruit puree over medium heat and warm for about 3 to 4 minutes without boiling.

In the bowl with the egg yolks, add the fine sugar and cornstarch. Whisk until the mixture turns pale and smooth, about 1 to 2 minutes.

Slowly pour half of the warm milk mixture into the egg yolk mixture while whisking continuously to temper the eggs.

Return the tempered egg mixture to the saucepan with the remaining warm milk and cook over medium heat, whisking continuously for approximately 5 to 7 minutes until thickened.

After thickening, remove from heat and stir in the unsalted butter until melted and glossy.

Transfer the filling to a bowl, cover with plastic wrap pressed onto the surface, and let cool at room temperature for about 30 minutes before refrigerating.