For the cake: Preheat the oven to 300 degrees F and butter a 9- by 13-inch baking pan.
Place the prunes in a small saucepan. Cover them with water. Bring to a low boil and cook until soft and mashable, about 8 minutes.
Drain the water and mash the prunes on a plate. It's okay to leave little chunks behind. Set the prunes aside and make the cake.
Sift together the flour, baking soda, allspice, cinnamon and nutmeg. Mmmm... smells like the holidays.
In a separate bowl, mix together the oil, sugar, vanilla and eggs.
Combine the wet and dry ingredients and splash in the buttermilk. Stir until just combined. The mixture will be slightly lumpy.
Now throw in the mashed prunes. And if your honey walks in as you're completing this step, shield the bowl with your body and stir quickly. What he doesn't know won't hurt him.
Pour into the prepared baking pan and bake for 35 to 40 minutes. Grandma Iny was adamant: do not over bake the cake. You want it to be nice and moist.
When the cake has about 5 minutes left to bake, make the icing: Combine the sugar, buttermilk, butter, corn syrup, baking soda and vanilla in a medium saucepan. Stir to combine. Bring to a slow boil over medium-high heat. No need to stir. 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.
Pull the cake out of the oven. Try not to faint, as it smells absolutely divine.
While the cake is very warm, pour the icing evenly over the top. Work fast, as it will quickly start to soak into the cake.
Spread the coat evenly ... then please, do yourself a favor: lick the spatula. It'll make you smile. Serve immediately, or feel free to let the cake sit on the counter for a while before serving. It only gets better with age.
Helpful hint: serve without revealing the fact that the cake contains prunes.
If you don't have any buttermilk, add 1 tablespoon distilled white vinegar to just under 1 cup low-fat milk and stir together.