Position a rack in the center of the oven and preheat the oven to 300ºF. (Custards need to cook slowly, so don't raise the temperature.) Lightly butter an 8-inch-square baking dish.
Whisk the eggs, sugar, vanilla, and salt together in a medium bowl. Gradually whisk in the milk. Scatter the rice and raisins in the baking dish, and pour in the custard. Sprinkle the cinnamon on top.
Bake until the pudding jiggles as a unit when gently shaken and a dinner knife inserted about 1 inch from the center comes out clean, about 45 minutes. Remove from the oven and let cool for about 10 minutes and serve warm. Or, cool completely, cover with plastic wrap refrigerate until cold, at least 2 hours. Serve chilled, with a drizzle of heavy cream on top, if desired.
