Mix flour, sugar, and salt in a large bowl. Add softened butter cubes and combine with an electric mixer until the mixture looks like wet sand. Slowly pour in cold water while mixing until the dough comes together but isn't sticky. Shape into a ball, wrap in plastic, and chill in the refrigerator for 30 minutes.

While the dough chills, warm the evaporated milk in the microwave for 1 minute until slightly warm (about 45°C/113°F). In a large bowl, whisk together 4 whole eggs and 1 egg yolk until well combined. Gradually pour in the condensed milk while whisking. Add vanilla extract and kalamansi juice, then mix in the warm evaporated milk until smooth. In a separate bowl, beat the remaining egg white until soft peaks form - when you lift the whisk, the peaks should gently fold over. Gently fold this beaten white into your main mixture using a sweeping motion, not stirring.

Preheat your oven to 350°F (175°C). Take out your chilled dough and roll it on a floured surface until it's about ½ centimeter thick and large enough to cover your 9-inch pie pan. Lift the dough carefully and place it in the pan, pressing gently into the corners. Trim the edges leaving a ½ inch overhang, then pinch or press with a fork to create a decorative edge. Pour your filling mixture into the crust.

Bake at 350°F (175°C) for 15 minutes, then lower the temperature to 325°F (163°C) and continue baking for 40-45 minutes. You'll know it's done when a toothpick inserted in the center comes out clean and the pie still has a slight jiggle. The top should be a beautiful golden brown. Let the pie cool completely at room temperature for at least an hour before slicing. During cooling, keep it away from drafts to prevent cracking.

If your pie browns too quickly during baking, tent the edges with foil. For the cleanest slices, refrigerate the cooled pie for an hour before cutting. Serve at room temperature with hot coffee or fresh milk. The pie will keep at room temperature for 2 days or in the refrigerator for up to 5 days.