Begin by preheating the oven to 350°F (177°C). Proceed to line a 12-cup muffin tray with paper liners, and lightly coat them with cooking spray. In a large bowl, combine the graham cracker crumbs, melted butter, ¼ cup of sugar, and the lightly beaten egg whites. Stir the mixture until it is uniformly mixed. Distribute approximately 3 heaping tablespoons of the crumb mixture into each muffin cup. Press the mixture firmly into the bottom and up the sides of each cup to form the crust.

Place the muffin tray in the preheated oven and bake the crusts until they are lightly browned and set, which should take about 12 minutes. After baking, transfer the pan to a wire rack and allow the crusts to cool completely for approximately 10 minutes. Following cooling, remove the crusts from the pan, discarding the liners, and then place the crusts on a large plate.

While the crusts are cooling, begin preparing the filling. In a medium saucepan, whisk together the eggs, egg yolks, granulated sugar, cubed butter, grated lemon zest, and fresh lemon juice. Cook this mixture over medium heat, stirring constantly to prevent the eggs from scrambling. Continue cooking until the mixture thickens, which should take about 5 minutes. Once thickened, pour the mixture through a fine-mesh strainer into a medium heatproof bowl to ensure smoothness.

To assemble the tartlets, divide the lemon curd evenly among the prepared crusts, adding about 2 tablespoons of curd to each crust. Cover the tartlets and refrigerate them until they are thoroughly chilled, which will take approximately 1 hour.

The Lemon Curd Tartlets are now ready to be served. Enjoy this exquisite balance of a buttery crust and a tangy lemon filling.