Using a fork, mix the cornstarch and lemon juice together in a small bowl until the cornstarch has dissolved. Set aside. Warm the blueberries and sugar together in a small saucepan over medium heat. Stir occasionally for 3 minutes, mashing the blueberries as best you can against the bottom and sides of the pan to help break them up. Stir in the cornstarch mixture. Cook for 2 more minutes as the sauce thickens, stirring and mashing the blueberries to break them up as desired. Remove sauce from heat and set aside at room temperature until step 6.

Preheat oven to 350°F (177°C).

Mix the melted butter, sugar, vanilla extract, and salt together in a medium bowl. Add the flour and stir to completely combine. The dough will be a little greasy and very thick. Press dough firmly into a 9-inch tart pan, making sure the layer of crust is even.

Bake for 15 minutes or until the edges are very lightly browned. Remove from the oven and poke a few holes all over the top of the warm crust.

Whisk all of the filling ingredients together until combined. Pour into warm crust. Stir up the blueberry sauce and drop spoonfuls of blueberry sauce all over the top, using about half of the sauce. Gently swirl the sauce and filling together.

Bake for 17-19 minutes, just until the center of the tart no longer jiggles when you tap the pan.

Remove tart from the oven, cool completely at room temperature, then chill in the refrigerator uncovered for at least 2 hours.

Slice and serve with optional garnishes including any leftover blueberry sauce.