Preheat the oven 400°F. Spray the cups of the mini muffin pans with baking spray.
For the tartlet shells: Roll out the chilled shortcrust pastry and cut rounds using a 2½-inch round cookie cutter. Using a pastry dough tart tamper, push the rounds into the muffin pan cups. Chill the dough for 15 minutes in the freezer or 30 minutes in the refrigerator to firm up. You don’t want slumped tartlet shells.
Partially bake the tartlet shells for 10 minutes. Remove them from the oven to cool while you make the filling.
Turn oven down to 350°F.
For the filling: Cut the lemon in half. Check if the thickness of the white ring of the skin (the pith) is larger than ¼-inch thick. If so, pare the zest from one half of the lemon, cutting away any of the pith before proceeding. Slice the lemon halves into thin slices, removing the seeds.
Put the entire slice (and any zest pieces), sugar, and butter into the container of a food processor. Purée the mixture, scraping down the sides of the container as needed, until the lemon is chopped fine. Add the eggs, cornstarch, and salt and pulse until the batter is smooth.
Use a small cookie scoop to portion 1 tablespoon of the batter into each of the pastry cups. Try not to pour it past the top of the tart shells or it will become difficult to unmold later.
Bake the tartlets for 25 to 30 minutes or until the filling is set (it will jiggle slightly and be very light brown on top).
Cool the tartlets in the pan placed on a wire rack for 10 minutes, then take them out of the pan to the wire rack. If necessary, use a tool that won’t damage your pan to pry the tarts out. Let the tartlets cool completely. Chill before serving if desired.
Dust with powdered sugar, then serve and enjoy.
If you have extra filling, you can pour the remainder into buttered ramekins and bake for about 20 minutes. Dust with powdered sugar after chilling. It’s quite yummy, like a cross between pudding and soufflé.Store the tartlets in the refrigerator. You can serve them at chilled or at room temperature, your call. They freeze well, and just need about 30 minutes on the counter to defrost.You can make one whole tart instead of the mini tartlets. Follow the directions above, pressing the shortcrust pastry into a 9-inch tart pan. Bake for 35 to 40 minutes until the filling is set (it will jiggle slightly in the middle and will be very light brown). Cool on a wire rack, then chill.