Part 1: Making and Blind-Baking the Sweet Tart Crust

1. Cream Butter and Sugar: In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric hand mixer, beat the cold, cubed butter and the sifted powdered sugar on medium speed until the mixture is pale, creamy, and fluffy.

2. Add the Egg: Add the beaten egg in two additions, mixing well after each one until fully incorporated. Scrape down the sides of the bowl.

3. Combine Dry Ingredients: Sift the cake flour and salt directly into the bowl. Mix on low speed until the dough just starts to come together into large crumbs. Do not overmix.

4. Form and Chill: Turn the dough out onto a lightly floured surface and gently bring it together with your hands into a flat disc. Wrap it tightly in plastic wrap and chill in the refrigerator for at least 1 hour (or up to 2 days). Chilling is crucial for resting the gluten and making the dough easy to handle.

5. Press into Molds: You will need 12 standard-sized (approx. 2.75-inch / 7cm diameter) metal tartlet molds. Lightly grease them. Divide the chilled dough into 12 equal portions. Working with one portion at a time, press the dough evenly into the bottom and up the sides of each mold. Use your thumb to create a neat, even edge that is flush with the top of the mold.

6. Dock and Freeze: Use a fork to prick the bottom of each tart shell a few times. This prevents the crust from puffing up during baking. Place the tart molds on a baking sheet and transfer to the freezer for at least 30 minutes. Freezing the shells helps prevent them from shrinking during the bake.

7. Blind Bake the Shells: Preheat your oven to 350°F (175°C). Place the frozen tart shells (still in their molds) on a baking sheet and bake for 15-20 minutes, or until the edges are lightly golden and the bottom looks dry and set. This is the first bake, known as “blind baking,” and it’s the secret to a crispy crust.

8. Cool Completely: Remove the shells from the oven and let them cool completely in their molds on a wire rack.

Part 2: Creating the Luscious Three-Cheese Filling

1. Beat the Cheeses: In a large bowl, using an electric mixer, beat the room-temperature cream cheese, mascarpone cheese, and finely grated Parmesan cheese on medium speed until completely smooth and free of lumps. This may take a couple of minutes. Scrape down the bowl frequently.

2. Add Sugar: Add the granulated sugar and beat until it’s fully incorporated and the mixture is smooth and creamy.

3. Incorporate Wet Ingredients: Add the 2 egg yolks, whole milk, fresh lemon juice, and vanilla extract. Mix on low speed until everything is just combined. Be careful not to overmix at this stage, as you don’t want to incorporate too much air.

4. Add Cornstarch: Sift the cornstarch over the mixture and fold it in gently with a spatula until no dry spots remain. The final filling should be smooth, thick, and pourable, similar to a thick pancake batter.

Part 3: The Assembly and Signature Double-Bake

1. Prepare for the Second Bake: Increase your oven temperature to 425°F (220°C) and position a rack in the upper third of the oven. A higher rack position helps the tops brown beautifully.

2. Fill the Shells: Transfer the cheese filling to a piping bag (or use a small spoon). Fill each of the completely cooled, pre-baked tart shells to the brim. The filling will puff up slightly and then settle, so you can even mound it a tiny bit above the rim.

3. Brush with Egg Wash: Prepare the egg wash by beating one egg yolk with one teaspoon of water. Using a pastry brush, gently and lightly brush the top of the cheese filling in each tart. This is the non-negotiable step for achieving that iconic, glossy, golden-brown top.

4. The Second Bake: Place the filled tarts on a baking sheet and bake in the hot 425°F (220°C) oven for 8-12 minutes. Keep a close eye on them. They are done when the filling has puffed up and the tops are beautifully golden brown, with some darker blistered spots.

5. Cool and Enjoy: Remove the tarts from the oven. Let them cool in their molds for about 10-15 minutes before very carefully removing them and placing them on a wire rack. They are extremely delicate when hot