Begin by separating the egg yolks from the whites and collecting all your ingredients. Measure out your whole milk, passion fruit puree, sugar, cornstarch, and butter.

In a medium saucepan, combine the whole milk and passion fruit puree. Place over medium heat and warm until fragrant, about 3-4 minutes, without boiling.

In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until pale. Gradually pour in half of the warm milk mixture, whisking constantly.

Return the mixture to the saucepan with the remaining milk blend. Cook over medium heat, whisking for about 5-7 minutes until thickened.

Remove from heat and stir in the unsalted butter until melted and the filling is glossy.

Transfer to a bowl and cover with plastic wrap pressed onto the surface to cool completely before using.