• Crust:
  • 6 tablespoons unsalted butter, at room temperature (3/4 stick)
  • 3 tablespoons sugar
  • Pinch fine salt
  • 1 large egg yolk
  • 3/4 cup all-purpose flour
  • 1 teaspoon sour cream or yogurt
  • Lemon Filling:
  • 1/3 cups sugar
  • 1 tablespoon cornstarch
  • 4 large egg yolks
  • 1/4 cup freshly squeezed lemon juice (about 2 lemons)
  • 2 tablespoons freshly squeezed lime juice (about 1 lime)
  • 1 tablespoon finely grated lemon zest
  • 3 tablespoons cold unsalted butter, cubed
  • Meringue:
  • 1/4 cup egg whites (about 2 large eggs)
  • 1/2 cup sugar
  • 1/4 teaspoon cream of tartar
  • Pinch fine salt
  • 1 (6-cup) standard non-stick muffin tin
  • Crust:

Beat the butter in a medium bowl with an electric mixer on medium-high until smooth. Add the sugar and salt and continue to beat until evenly combined. Scrape down the sides of the bowl, then beat in the egg yolk. Add half of the flour, beating until just crumbly. Scrape down the bowl again; add the remaining flour and then the sour cream or yogurt, beating just until the dough is evenly moistened. Turn dough onto a lightly floured work surface and knead lightly to bring it together. Roll the dough between 2 generously floured sheets of waxed or parchment paper into a circle about 1/4-inch thick with a rolling pin. Refrigerate for 1 hour. Cut out 6 (4 1/2-inch) rounds using an inverted bowl or round cookie cutter. Place rounds in the muffin tins and, using a small shot glass or your fingers, press into the corners and about halfway up the sides for a snug fit (see photo). Freeze dough in the muffin tin for 30 minutes. Position a rack in the lower third of the oven and preheat to 325 degrees F. Spray the outside of 6 standard muffin liners with cooking spray and place in the crusts. Fill with dried beans or pie weights. Bake crusts until just brown around the edges, about 25 to 30 minutes. Remove from the oven and cool for about 1 minute; then carefully remove the muffin liners and baking beans. Return pan to the oven and continue to bake until crusts are cooked through and evenly browned (see photo), about 15 to 20 minutes more. Cool slightly. Then carefully remove crusts from the muffin tin and cool completely on a rack.