The defining feature of this dessert is that it has two layers. The top layer is a delicate, just-set custard, a little like a crème caramel (minus the caramel poured over the top), while the bottom is a firmer layer of soft rice, bound with custard. Each bite is reminiscent of rice pudding and crème caramel all at once.
Note: This mixture is extremely runny. Do not use a springform or loose-bottomed cake tin: You will end up with the raw mixture dripping (or running!) all over your oven. The recipe is adapted from Paolo Petroni's "Il Grande Libro della Cucina Toscana." —Emiko
Prepare a 9- or 10-inch (23- or 25-cenimeter) solid-bottomed round cake pan or pie dish by greasing well with butter and sprinkling with sugar to cover (if not making a gluten-free dessert, you can sprinkle with a dusting of flour instead of sugar).
Cook the rice with a pinch of salt in boiling water, as you would pasta. Remove after about 10 minutes—it should be al dente.
In a large mixing bowl, whisk together the eggs, sugar, rum, zest, vanilla, and a pinch of salt until combined. Add the warmed milk and the rice.
Pour the custard into the prepared pan. The rice should settle evenly along the bottom. Bake at 350° F (180° C) for about 50 to 60 minutes, or until the top is golden brown with some darker splotches and firm, not liquid, when gently touched (a skewer inserted into the middle should come out clean).
Remove and let cool in the pan. Serve cold or at room temperature and store any leftovers in the refrigerator, covered with plastic wrap, for 3 days at most.