Instructions

• In a food processor combine the 1 1/4 cups all-purpose flour, 3 1/2 teaspoons powdered sugar, 1/4 teaspoon salt, and the 11 tablespoons cold, cubed unsalted butter. Pulse until the mixture is crumbly and the butter pieces are slightly larger than peas.
• Transfer the crumb mixture to a bowl. Add the 3 1/2 ounces sour cream (plus additional sour cream, 1 tablespoon at a time, only if the dough seems too dry). Gently fold by hand until the dough just comes together; use your hands to bring it into a loose ball without overworking.
• Wrap or cover the dough and refrigerate for 10 minutes to firm.
• Place the chilled dough between two pieces of parchment paper. Roll it into a square about 1/4 to 1/2 inch thick.
• Fold the dough into thirds: fold the left third over the center, then fold the right third over the left (like a letter). Cover and freeze for 10 minutes.
• Remove the dough, turn it so the long side is facing you, and roll it again into a 1/4 to 1/2 inch thick square. Fold into thirds the same way, then return the dough to the freezer for 15 minutes.
• Grease a standard metal muffin pan (or use silicone molds). Roll the dough one final time to about 1/4-inch thickness between parchment sheets. Use a round cookie cutter to cut circles sized to fit your muffin wells, then press each circle into a well to form a tart shell. Pierce the bottom of each shell several times with a fork.
• In a bowl whisk together the 4 egg yolks, 1/2 cup evaporated milk, 1/4 cup condensed milk, and 3/4 teaspoon vanilla extract until smooth. Strain the mixture through a fine-mesh sieve into a clean bowl to remove any lumps.
• Preheat the oven to 400°F. Pour the strained custard into the prepared tart shells, filling each about 3/4 full.
• Bake at 400°F for 10 minutes. Without removing the pan from the oven, reduce the oven temperature to 350°F and bake an additional 5–10 minutes, until the custard is set in the center and the edges are golden brown.
• Let the tarts cool in the pan for a few minutes, then unmold them and place on a rack to cool at room temperature for 1–2 hours before serving.