Make the crust: Place the flour, sugar, baking powder, eggs, melted butter and milk in a large bowl. Beat with a mixer on medium speed until smooth, 4 to 5 minutes.
Lightly butter a 10-by-15-inch baking dish. Press the dough into the bottom and up the sides of the pan in an even layer; set aside.
Make the filling: Preheat the oven to 350 degrees F. Beat the butter and sugar in a large bowl with a mixer on medium speed until combined, 2 to 3 minutes. Add the ricotta, heavy cream, eggs and vanilla and beat until fully incorporated, scraping down the bowl with a rubber spatula. Fold in the rice and orange zest with the rubber spatula.
Pour the filling over the prepared crust. Bake until the edges are golden brown and set but the center is still slightly jiggly, 1 hour 45 minutes to 2 hours 15 minutes. Let cool before slicing.