Lightly spray a mini muffin tin with nonstick cooking spray. Set aide. Place rack in middle of oven and preheat oven to 325 degrees. Using one crust at a time, unroll crust on a lightly floured work surface. Using a 2 1/2 inch round cutter, cut out as many circles as possible. Re-roll remaining dough and continue to cut more circles until all dough is used up. Press circles into muffin tin. Place remaining circles and muffin tin in refrigerator to chill for 15-20 minutes.
Remove muffin tin from fridge. Prick bottoms of shells with a fork. Place a mini muffin liner filled with beans in each muffin cup. Bake the shells for 20 minutes. Let cool for 5 minutes and then remove the cups of beans. Place shells on a rimmed baking sheet. Repeat with remaining dough until finished.
For the filling: In a medium saucepan, whisk together sugar and cornstarch. Whisk in 1 cup of water and bring to a boil over medium heat, stirring often until mixture starts to thicken. Place the egg yolks in a bowl. Add a ladleful of the hot sugar mixture, whisking as your adding. Pour the yolk mixture into the saucepan and continue whisking until well combined. Turn the heat to low and continue to whisk for 1 minute. Remove from heat and add the lemon zest, lemon juice and butter and whisk until combined. Fill the shells with a spoonful of the lemon filling.
Preheat oven to 375 degrees. For the meringue: In the bowl of a stand mixer fitted with the whip attachment, beat egg whites, cream of tartar and salt on medium-high speed until soft peaks form. Slowly add the sugar and continue to beat until stiff peaks form, 1-2 minutes. Top the filling with a dollop of meringue spreading to the edge of the shells. Bake for 10-12 minutes until golden brown. Remove from oven and let cool completely on wire rack before serving.
You can also use a kitchen torch to brown the meringue a bit more or place under the broiler. Keep a close eye so they don't burn.