Place the flour, sugar, salt, baking powder and citrus zest in the bowl of your food processor and pulse to combine all dry ingredients.
Add butter and pulse until the mixture resembles coarse sand and there are no large pieces of butter.
Add the eggs to the food processor and pulse until the dough comes together.
Transfer the dough to a lightly floured surface and knead by hand to make sure there are no dry spots. Form the dough into a ball, flatten into a disc, wrap in plastic and chill until firm, 1 to 2 hours.
Place the cooked grain, milk, sugar, vanilla bean and pod, zests, and cinnamon in a saucepan and cook over low heat, stirring often with a wooden spoon until the mixture is creamy and looks like oatmeal. Set aside and allow it to cool. Remove vanilla bean pod, rinse, and set aside for another use.
For the crema pasticcera, fill a large bowl halfway with ice and a little water. Set a stainless steel or glass bowl on top of the ice.
In a saucepan set over medium heat, heat the 2 cups of whole milk.
In a separate bowl, whisk together the ½ cup sugar and cornstarch. Once combined, whisk in the eggs.
When the milk comes up to a simmer, slowly pour the milk into the egg mixture making sure to continuously whisk. Pour the mixture back into the pan and whisk over medium heat until the mixture thickens and holds the lines of the whisk.
Pour the crema pasticcera into the bowl set on top of the ice bath. Whisk until cool.
Between two sheets of plastic crap, roll about two thirds of the pasta frolla into a 12-inch circle, 1/8th of an inch thick. Peel off the top layer of plastic and flip the dough over and transfer the dough to a deep, 9-inch pie plate. Remove the second layer of plastic.
Chill the shell while you put together the rest of the filling.
Whisk together the eggs, ricotta, vanilla, and orange flower water. Mix in the grain mixture and candied peel. Fold in the chilled crema pasticcera.
Roll out the remaining dough to 1/8” thickness and cut ¾” strips from the dough.
Pour the filling into the chilled shell and carefully arrange the enough of the strips on top to form a lattice. Press the edges together with the overhang to seal. Trim the excess dough.
Use a fork to slightly beat the egg with a little water to make an egg wash. With a brush flaze the lattice topping.
Bake the pastiera on the middle rack of the oven for 55 to 60 minutes.
The pastiera is done when the filling is set but jiggly and the pastry is golden brown. Allow the pastiera to cool completely, or chill it overnight. Serve chilled or room temperature.