Preheat oven to 300 degrees. Heat butter in a 4 quart saucepan. When frothy add rice and cook for a minute. Slowly add milk, pinch salt, sugar, vanilla, cranberries and bring to under a boil. Transfer to a 9 inch square baking pan and set in oven. Bake uncovered for 1 1/2 hours stirring every 15 minutes. Then bake undisturbed for 30 minutes. Then stir in cranberries, stir and bake for another half hour (for a total of 2 1/2 hours).
Variations: you can to add heavy cream in the last half hour of baking. Or if you want a crusty topping, reduce sugar to 1/4 cup. Once out of oven, crumble brown sugar and butter over rice pudding and run the pudding under the broiler until melted and bubbly.