Special equipment: four 15-ounce cast-iron melting pots, optional (you can also make 1 large cake in a 9-by-13-inch pan)
For the cake: Preheat the oven to 300 degrees F and butter four 15-ounce cast-iron melting pots (or a 9-by-13-inch pan if using instead).
Sift together the flour, baking soda, allspice, cinnamon and nutmeg in a bowl. In a separate bowl, mix together the oil, sugar and eggs. Combine the wet and dry ingredients in one bowl and splash in the buttermilk. Stir until just combined. The mixture will be slightly lumpy.
Pour into the prepared pots (or 9-by-13-inch pan if using) and put them on a baking sheet. Bake until a skewer inserted in the middle comes out clean, 40 to 45 minutes; do not over bake. (If using a 9-by-13-inch pan, bake for 25 to 30 minutes.)
For the icing: While the cake is baking, make the icing. Combine the sugar, buttermilk, butter, corn syrup, baking soda, vanilla and salt in a medium saucepan. Stir to combine. Bring to a slow boil over medium-high heat without stirring. Continue boiling until the icing turns a light caramel color, 5 to 7 minutes. Important: The icing should be the color of caramel, but not yet firm and sticky. It needs to be pourable.
Remove the cakes from the oven. While they are very warm, pour the icing evenly over the tops. Work fast, as it will quickly start to soak into the cakes. Serve with ice cream if desired.