Preheat oven to 300 degrees. Butter a 9 x 13 inch baking dish and set aside.
Place prunes in medium sauce pan and cover with water. Bring to a boil and cook until soft, approximately 8 minutes. Remove from heat, drain water, and mash on a plate. Set aside.
Combine oil, sugar, and eggs. In a separate bowl, sift together dry ingredients. Combine wet and dry ingredients, add buttermilk and vanilla and stir gently until just combined. Add mashed prunes and stir gently Pour batter into buttered baking dish and bake for 35 to 40 minutes.
When cake has five minutes remaining, make the icing: Combine all icing ingredients in a medium saucepan over medium heat and bring to a slow boil. Boil without stirring for 5 to 7 minutes, or until icing starts to turn dark. Do NOT allow icing to reach soft ball stage; icing should be caramel in color, but not sticky like caramel. Icing should pour easily.
Remove cake from oven and pour on icing immediately. Allow to rest on the counter. Serve warm.
Note – If you are not able to serve immediately, you can reheat the entire cake in a 200 degree oven for approximately 5 minutes. Individual slices reheat in microwave for 10 seconds.