Preheat oven to 300 F. Grease or spray a 13x9 baking pan.
Place prunes in a medium saucepan and cover with water. Bring to a boil and cook for 7-8 minutes. Remove from heat, drain, reserving 2 tablespoons of the boiling water and place prunes, vanilla and reserved water into a mini food processor. Puree and then set aside.
In a large bowl, mix together the sugar, eggs and canola oil, beating well.
In a separate bowl, whisk together the flour, cocoa, and cinnamon.
To the large bowl with the egg/oil mixture add all ingredients (flour mixture, buttermilk, and prune puree) and stir until well combined.
Scrape batter into prepared pan and bake for 35-40 minutes.
Five minutes before cake has finished baking, make the icing. Combine all icing ingredients in a medium saucepan. Bring to a gentle boil over medium heat. Once icing begins to boil, give it a quick few stirs to combine everything then let it boil, undisturbed for 6-7 minutes.
Remove cake from oven to a cooling rack. Remove icing pan from heat and give it one or two stir then pour over the top of the cake. Let cake cool for 10 minutes and serve warm.
Notes
This cake is delicious once cooled too, but I found that my family preferred heating it in the microwave and serving it with vanilla ice cream.