Begin your tropical journey by separating the egg yolks from the whites in a bowl. Measure out all ingredients, including whole milk, passion fruit puree, sugar, cornstarch, and unsalted butter. Having everything ready will streamline your process.

In a medium saucepan, combine the whole milk and passion fruit puree over low to medium heat. Gently warm the mixture until it becomes steamy, about 3-5 minutes. Stir occasionally to prevent sticking.

In a separate bowl, whisk together the egg yolks, fine sugar, and cornstarch until smooth. Pour half of the warm milk mixture into the bowl while whisking continuously. Return this mixture to the saucepan.

Place the saucepan back on medium heat and cook the mixture, whisking constantly for about 5-7 minutes until it thickens and bubbles. Cook for an additional minute while stirring.

Remove the saucepan from heat and add the unsalted butter, stirring until completely melted for a glossy finish.

Transfer the filling to a clean bowl and cover it with plastic wrap, pressing it onto the surface to prevent a skin from forming. Allow it to cool completely before use.