Step 1: Preparing the Flaky Shortbread Crust In a large mixing bowl, whisk together the flour, sugar, and salt. Drop in the cold butter cubes. Using a pastry cutter or your fingertips, work the butter into the flour until the mixture looks like coarse sand with a few pea-sized butter lumps remaining. This is the secret to a flaky, tender crust. In a small bowl, beat the egg yolk with 2 tablespoons of ice water. Drizzle this over the flour mixture and stir with a fork until the dough just starts to come together. If it feels too dry, add the remaining tablespoon of water. Press the dough into a ball, flatten it into a disc, wrap it in plastic, and chill it in the refrigerator for at least 30 minutes.
Step 2: Pressing and Shaping the Tart Shells Once the dough has chilled, lightly flour your work surface. Roll the dough out to about 1/8-inch thickness. Using a round cutter or a small bowl, cut out circles that are slightly larger than your 4-inch tart pans. Gently press each circle into the tart pan, making sure to push the dough into the fluted edges without stretching it. Run your rolling pin over the top of the pan to trim off any excess dough. Prick the bottom of each shell a few times with a fork to prevent air bubbles. Place the shaped shells back in the fridge for another 20 minutes to firm up—this is your insurance against a shrinking crust.
Step 3: Blind Baking the Crust to Golden Perfection Preheat your oven to 375°F (190°C). Place the chilled tart shells on a baking sheet. Line each shell with a small piece of parchment paper and fill with pie weights, dry beans, or rice. Bake for 12 minutes. Remove the parchment and weights carefully, then bake for another 5 to 7 minutes, until the bottoms are lightly golden and the edges are set. Let the shells cool completely on a wire rack while you make the fillings.
Step 4: Whisking the Luscious Lemon Curd In a medium saucepan, whisk together the eggs and sugar until smooth and pale. Whisk in the lemon juice and lemon zest. Place the saucepan over medium-low heat, stirring constantly with a whisk or a wooden spoon. Do not walk away! Cook for about 6 to 8 minutes, until the curd thickens enough to coat the back of the spoon. It should look like a thick, creamy custard. Immediately remove it from the heat and whisk in the cold butter pieces, one at a time, until the curd is glossy and smooth. Pour the curd through a fine-mesh strainer into a clean bowl to catch any cooked egg bits or zest. Press a piece of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Let it cool to room temperature, then refrigerate for at least 20 minutes to set.
Step 5: Simmering the Sweet Blueberry Jam In a small saucepan, combine the blueberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the blueberries burst and release their juices, about 5 minutes. Give the cornstarch slurry a quick stir and pour it into the bubbling berries. Stir constantly for another 1 to 2 minutes, until the mixture thickens into a glossy, jam-like consistency. Remove from heat and let it cool completely. The jam will continue to thicken as it sits.
Step 6: Assembling the Perfect Tart Once the tart shells, lemon curd, and blueberry jam are all completely cool, it is time to build. Spoon or pipe the chilled lemon curd into each tart shell, filling them almost to the top. Smooth the surface with the back of a spoon. Gently spoon the blueberry jam over the lemon curd, spreading it evenly. You can leave a little border of yellow curd showing if you like the two-tone look. Pop the assembled tarts into the refrigerator for about 30 minutes to let everything set together.
Step 7: Garnishing and Serving Your Masterpiece Just before serving, dust the tarts lightly with powdered sugar using a small sieve. Add a tiny pinch of fresh lemon zest on top for a pop of color, and place a mint leaf next to each tart for a fresh green accent. Serve them cold or at room temperature. The contrast of the cool, creamy curd against the sweet berry jam is nothing short of spectacular.