Lightly butter or spray with nonstick vegetable spray 36 miniature muffin tins (approximately 2 inch/5 cm in diameter). Set aside. Preheat oven to 325 degrees F (170 degrees C) and place rack in the center of the oven.

In the bowl of your electric mixer, or with a hand mixer, beat the butter and sugar together (approximately two minutes). Beat in the vanilla extract. Add the flour, cornstarch and salt and mix 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 18 - 20 minutes or until lightly browned. About halfway through the baking time, lightly prick the bottom of each shortbread, with the tines of a fork, if they have puffed up. 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.

In your food processor or 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 bowl, cover, 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.)

Garnish with any combination of berries (blueberries, strawberries, blackberries, raspberries) or other fruit such as slices of kiwi.