Blueberry and Vanilla Custard Tart

Preheat the oven to 350 degrees F (175 degrees C). Lightly butter a 9-inch tart pan with a removable bottom.

Mix flour, sugar, cinnamon, cream of tartar, and salt for crust together in a medium bowl until combined. Mix in melted butter, egg yolk, and vanilla until combined. Firmly press 2/3 of the mixture into an even layer in the bottom of the prepared tart pan. Sprinkle remaining 1/3 of the mixture around the edges, then press firmly and evenly into the sides of the pan, creating a slightly raised edge over the top. Lightly prick the bottom of the crust with a fork several times, not going all the way through.

Mix egg white and water together until combined. Brush a light layer of egg white over the bottom of the crust. Place the tart pan onto a baking sheet.

Bake in the preheated oven just until the crust begins to brown, 25 to 27 minutes. Remove from the oven and allow crust to cool for 15 minutes. Leave the oven on.

While crust is cooling, make the custard. Whisk together egg yolks, sugar, and salt until thick and light in color. Mix in vanilla extract. Gradually add heavy cream, mixing gently but thoroughly, to avoid incorporating too much air. Carefully pour custard into the cooled crust. (You may or may not have extra; just don't overfill.)

Place tart back into the oven and bake until custard is just set and lightly browned, about 30 minutes. Remove from the oven and cool to room temperature, at least 1 hour.

While tart cools, make blueberry topping. Place blueberries and sugar in a medium saucepan over medium heat. Cook, stirring occasionally, until berries begin to burst and release their juices, about 15 minutes. Combine lemon juice and cornstarch in a small bowl or measuring cup. Add cornstarch mixture to the blueberries, stirring constantly, and cook until thickened, 1 to 2 minutes. Remove from heat and allow to cool to room temperature.

When both the tart and the blueberries have cooled, carefully spoon blueberries over the top of the tart, and spread gently into an even layer. Place tart into the refrigerator and chill for at least 2 hours. Remove sides of tart pan, slice, and serve.