1. Blueberry Sauce:
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. Makes about 1/2 – 2/3 cup blueberry sauce and you’ll use about half for the swirl. (Reserve extra for garnish/serving.)
2. 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.
Using a rubber spatula or your hands, press dough firmly into a 9-inch tart pan (no need to grease it), making sure the layer of crust is even on the bottom and all around the sides. It may seem like it won’t fit, but it will. You can use the bottom of a measuring cup to pack the crust in too, but sometimes it sticks.
4. Bake for 15 minutes or until the edges are very lightly browned. Remove from the oven. Using a fork, poke a few holes all over the top of the warm crust (not all the way through the crust). This helps the filling stick.
Whisk all of the filling ingredients together until combined. Pour into warm crust. Stir up the blueberry sauce because it has likely thickened. It can still be warm when you use it in this step. If it’s too thick, warm it in the microwave for 5-10 seconds.
Drop spoonfuls of blueberry sauce all over the top, using about half of the sauce. Reserve the rest of the sauce for serving. Using a toothpick or knife, gently swirl the sauce and filling together. Shimmy the pan back and forth 2-3 times to let the sauce and filling settle down into the crust a bit.
6. Bake for 17-19 minutes, just until the center of the tart no longer jiggles when you give the pan a light tap. It will still be a little sticky on top. Avoid over-baking because the tart will taste rubbery.
7. Remove tart from the oven and place on a cooling rack. Cool completely at room temperature, then chill in the refrigerator uncovered for at least 2 hours and up to 1 day. If chilling for longer than 2 hours, cover it.
After chilling, remove the sides of the tart pan if your pan has removable sides.
8. Slice and serve with optional garnishes including any leftover blueberry sauce.
9. Cover leftovers tightly and refrigerate for up to 1 week.
Make Ahead Instructions: You can bake, cool, and chill the tart 1 day in advance. See step 7. The filling must be poured on a warm crust so it will stick, so I don’t recommend pre-baking the crust ahead of time. You can prepare the filling and blueberry sauce in advance though. Cover both and refrigerate until ready to use. The blueberry filling will be quite solid after refrigerating, so warm it up on the stove or in the microwave until thinned out again. (It can be warm when you swirl it into the filling.) Baked and cooled tart freezes well up to 3 months. Thaw overnight in the refrigerator before serving.
Tart Pan: A 9-inch tart pan with or without removable sides (it doesn’t matter) is ideal for this recipe. If you use other size tart pans, the bake times and thickness of the tart will differ. You can also use a 9-inch pie dish, but slicing the tart isn’t quite as neat. The bake times are the same.
Blueberry Sauce: If you don’t want blueberry lumps in your tart, feel free to run the blueberry sauce through a fine mesh sieve.
Sweetened Condensed Milk: Make sure you are using sweetened condensed milk, not evaporated milk. There are no successful replacements for sweetened condensed milk in this particular recipe– it’s really the key ingredient. (Sometimes half-and-half or heavy cream works as a substitution, but then you’ll have to test with varying amounts of sugar since both are unsweetened.) Use one 14 ounce can full-fat sweetened condensed milk. Do not use fat free because the tart will not set. Keep in mind that 14 ounces is the listed weight on the can. 1 standard 14 ounce can is a little over 1 liquid cup.