Preheat your oven to 350°F (175°C). Grease a 9-inch tart pan with a removable bottom.

In a small bowl, mix flour, cornstarch, and salt. Set this mixture aside.

In the bowl of a stand mixer, beat the butter and powdered sugar until the mixture is light and fluffy. Add the egg and mix until it is combined.

Gradually add the flour mixture and mix until it is barely combined. Knead the dough into a ball, wrap it in plastic, and refrigerate for 1 hour.

Roll out the chilled dough to a 1/4-inch thickness. Line the tart pan with the dough and chill it while the oven preheats.

Line the crust with foil and pie weights, and bake for 10 minutes. Remove the foil and weights, then bake for another 10 minutes until the crust is golden.

For the blueberry curd, combine blueberries and water in a saucepan. Bring this mixture to a boil, then simmer until the blueberries burst, about 5 minutes. Strain the mixture to get just the juice.

In the same saucepan, whisk the blueberry juice with the eggs, egg yolks, sugar, lemon juice, and cornstarch over low heat. Add the diced butter piece by piece, stirring until the mixture thickens, taking about 5-8 minutes.

Strain the curd to remove any solids, then pour it into the baked tart shell. Bake the tart for 15 minutes.

Let the tart cool to room temperature, then chill it in the refrigerator overnight.

Serve it topped with more fresh berries.