Preheat the oven to 325F and lightly grease a 10-inch ceramic tart pan. (You can use a metal pan too but watch it more carefully as the crust will bake and brown faster).
Prepare the crust as instructed, and press firmly into the bottom and up the sides of the pan. Prick all over with a fork and bake 15 to 20 minutes, until golden brown. Remove and let cool completely.
While the crust is baking, prepare the lemon curd.
In a glass or ceramic bowl set over a pan of barely simmering water, whisk together the eggs, sweeteners, lemon juice, and lemon zest. Whisk continuously until the mixture thickens, about 10 minutes. Watch carefully as it will thicken all of a sudden.
Immediately remove from heat and add the butter. Let sit a few minutes to melt and then whisk until smooth and creamy. Let cool at least 15 minutes.
In a large bowl, beat the mascarpone with the powdered sweetener until well combined. Beat in the lemon zest and lemon extract.
Add ½ cup of the cooled lemon curd and beat until smooth. Spread the filling in the cooled crust. Freeze 1 hour to set.
Spread the remaining lemon curd over the filling and chill another 3 hours to set before serving.