Lightly butter, or spray with a nonstick vegetable spray, 36 miniature muffin tins (approximately 2 inch in diameter). Set aside. Preheat oven to 325 degrees F and place rack in the center of the oven.
In the bowl of your electric mixer, beat the butter and sugar together (approximately two minutes). Beat in the vanilla extract. Add the flour, cornstarch and salt and mix just until incorporated.
Divide the dough into 36 even pieces and place one ball of dough in the center of each muffin tin. With your fingertips, press the dough up the sides of the individual muffin tin so there is an indentation in the center. Once filled, place the tart pan, with the unbaked shells, in the freezer for about 10 minutes so the shortbread can become firm. (This will help to prevent the shortbread from puffing up during baking.)
Bake for approximately 15 - 18 minutes or until lightly browned. About halfway through the baking time, lightly prick the bottom of each shortbread, with the tines of a fork. Check again after another five minutes and prick again if needed. Once they are fully baked, remove from oven and place on a wire rack to cool. When completely cooled, remove the tarts from the pan.
Lemon Creamcheese Filling
In your electric mixer, beat the cream cheese until fluffy. Add the condensed milk, lemon juice, zest, and vanilla and process just until smooth. (Do not over process or the filling will be too runny.) Transfer the filling to a large Ziplock bag and refrigerate until serving time. (It is best to make the filling at least a day in advance to give it time to become firm and for all the flavors to mingle).
Cut a corner off of the Ziplock bag and fill the tart shells with the cream cheese filling. The filled tart shells can be made several hours in advance and refrigerated. Just before serving top with any combination of fresh raspberries, blackberries, strawberries, blueberries or other sliced fruit such as kiwi. You may also sprinkle fruit with toasted chopped nuts, toasted coconut or shaved chocolate bits.