In a medium saucepan, heat the milk and lemon zest over medium heat until just about to boil. Remove from heat, cover, and let steep for 10 minutes.
In a bowl, whisk together the egg yolks and sugar until pale and creamy.
Add the cornstarch and whisk until smooth and lump-free.
Remove the lemon zest from the milk, then slowly pour the warm milk into the egg mixture, whisking constantly to temper the eggs.
Return the mixture to the saucepan. Cook over medium heat, stirring constantly, until thickened (about 3 to 5 minutes). The cream should coat the back of a spoon.
Remove from heat, stir in vanilla extract, and transfer to a bowl.
Cover with plastic wrap pressed directly onto the surface to prevent a skin from forming. Let cool completely, then refrigerate until cold.
2. Prepare the pastry dough (pasta frolla)
In a large bowl, combine flour, sugar, salt, and lemon zest.
Add cold butter cubes and rub them into the flour mixture using your fingertips or a pastry cutter until it resembles coarse crumbs.
Add the egg yolks, egg, and vanilla extract. Mix gently until a dough begins to form.
Gather the dough into a ball, flatten into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour (or up to overnight).
When ready to assemble, divide the chilled dough into two portions: one slightly larger for the base, and a smaller one for the top.
3. Assemble the tart
Preheat oven to 350°F (180°C).
On a lightly floured surface, roll out the larger portion of dough into a circle large enough to line a 9-inch (23 cm) tart pan with a removable bottom.
Gently press the dough into the pan, trimming off any excess and patching any cracks.
Spoon the chilled pastry cream evenly into the crust and smooth the surface.
Roll out the remaining dough and gently place it over the filling. Seal the edges by pinching or pressing gently with your fingers.
Sprinkle the pine nuts evenly over the top, pressing them lightly so they adhere.
4. Bake
Bake the tart for 40 to 45 minutes, or until the top is lightly golden and the crust edges are firm.
If the pine nuts begin to brown too quickly, cover loosely with foil.
Remove from oven and allow the tart to cool completely in the pan on a wire rack.
5. Finish and serve
Once cool, remove the tart from the pan carefully.
Just before serving, dust generously with powdered sugar.
Slice and serve at room temperature, or slightly chilled.
Storage
Refrigerator: Store covered in the refrigerator for up to 3 days.
Freezer: You can freeze the baked tart (without powdered sugar) for up to 2 months. Thaw overnight in the refrigerator and dust with sugar before serving.
Tips and Variations
For a stronger citrus note, add a bit of orange zest along with the lemon.
Replace part of the milk with cream for an extra rich filling.
Some regional variations place the pine nuts under the top pastry layer instead of on top.
To make a single-crust version (without the top layer), fill the pastry with custard, top with pine nuts, and bake uncovered. This version is common in northern Italy.
Serving Suggestions
Serve Torta della Nonna with a cup of espresso or vin santo. It’s a dessert that improves in flavor after resting for a few hours, as the custard and crust meld together beautifully.
