For the crust: In the bowl of a food processor, pulse together the flour, sugar, and salt until combined. Add the butter and pulse until the mixture resembles coarse cornmeal. While continuing to pulse, slowly add the water and lemon juice until the dough just comes together. Squeeze a small amount of dough between your thumb and index finger to make sure it holds its shape. If it doesn't, add cold water 1 tablespoon at a time while still pulsing, but make sure that the dough doesn't form into a wet ball.
Remove the dough from the food processor. Divide the dough in half, press each half into a disk, and wrap with plastic wrap. Refrigerate the dough for 20 minutes. (You will only use 1 disk of dough for this recipe; the other disk of dough will keep for up to 1 month, wrapped well, in the freezer.)
Preheat the oven to 350 degrees F.
On a lightly floured work surface, roll 1 dough disk out to a 12-inch round. Line a 9-inch pie pan with the dough. Crimp the edges and dock the bottom of the dough with a fork. Freeze the dough in the pan for 20 minutes.
Line the dough-lined pan with parchment paper and fill to the rim with dried beans or pie weights. Bake for 20 minutes. Remove the weights and parchment. Continue baking until the crust is baked through, 15 to 20 more minutes. Let cool completely.
For the filling: In the bowl of a stand mixer fitted with a whisk attachment, combine the egg yolks, maple syrup, cornstarch, and salt, whisking until smooth and pale.
In a large, heavy saucepan, combine the cream, milk, and vanilla and bring to a simmer. With the mixer running on medium speed, slowly and carefully pour the hot milk mixture into the mixer and whisk until combined.
Transfer the mixture back to the saucepan and whisk constantly over medium-low heat until the mixture thickens to the consistency of mayonnaise. Remove the pastry cream from the heat and add the butter, whisking until incorporated. Pour the pastry cream directly into the cooled prepared crust. Cover the tart with plastic wrap and refrigerate until cool.
To finish: In the bowl of a stand mixer fitted with the whisk attachment, whisk the heavy cream, mascarpone, and maple syrup to stiff peaks. Using a small offset spatula, spread the topping across the surface of the pie.
Garnish each slice with a Maple Cookie, if you like.
Preheat oven to 350 degrees F. Line 2 sheet pans with parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter, maple syrup, and 1/2 cup of the sugar until light and fluffy.
In a large mixing bowl, whisk together the flour, cornstarch, and salt until fully combined. Add the flour mixture all at once to the butter mixture and mix on low speed until just incorporated. Turn the dough out onto a sheet of plastic wrap. Press the dough into a disk and wrap it up. Refrigerate for 1 hour.
On a floured work surface, roll out the dough to 1/8 inch thick. Using a maple leaf cookie cutter, stamp out cookies and place 1/2 inch apart on the prepared pans. Press together and re-roll the scraps 1 time. Sprinkle the cookies with a light, even layer of the remaining 1/4 cup sugar.
Bake until the edges of the cookies are golden brown, 15 to 17 minutes.