Begin by mixing the all-purpose flour, powdered sugar, and salt in a mixing bowl. Add the cold, cubed unsalted butter and use a pastry cutter or your fingers to work the butter into the flour mixture until it resembles coarse crumbs.
Stir in the sour cream. If the dough seems too dry, add a little more sour cream, a teaspoon at a time, until you can form the dough into a ball.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes. This chilling step is crucial for achieving a flaky crust.
In a separate bowl, whisk together the egg yolks, evaporated milk, condensed milk, and vanilla extract until smooth. Make sure there are no lumps in your mixture, as a smooth filling is key to a perfect egg tart.
Preheat your oven to 375°F (190°C) while the dough is chilling.
Once chilled, take the dough out of the refrigerator and roll it out on a lightly floured surface. Aim for a thickness of about 1/8 inch. Cut out circles that are slightly larger than your tart pans to ensure you have enough dough to cover the sides.
Gently press the dough circles into the tart pans, ensuring they are evenly distributed along the bottom and sides. Prick the bottom of each crust with a fork to prevent bubbling during baking.
Bake the crusts in the preheated oven for about 10–12 minutes until they are just starting to turn golden. Remove them from the oven and let them cool slightly.
Once the crusts are slightly cooled, pour the egg filling into each tart shell, filling them about 3/4 full to allow for some expansion during baking.
Return the tarts to the oven and bake for an additional 15–20 minutes or until the filling is set but still slightly jiggly in the center. Keep an eye on them to avoid overbaking.
Allow the egg tarts to cool at room temperature before transferring them to the refrigerator to chill completely. Serve them cold or at room temperature for the best flavor.