Preheat oven to 375. In a large mixing bowl, whisk together flour, sugar, baking soda, salt and cinnamon, set aside. Melt butter in a saucepan then stir in applesauce, cocoa powder and water and bring mixture just to a boil, whisking constantly. Pour hot butter mixture into flour mixture and stir.
Add in buttermilk, eggs and vanilla and whisk mixture until well combine. Pour mixture into a buttered 13x9 baking dish and bake 25-28 minutes until toothpick inserted in center comes out with a few moist crumbs attached, prepare frosting during the last 5 minutes of cake baking.
Immediately after removing cake from oven, pour (or dollop frosting by the spoonfuls) over hot cake and gently spread over entire cake.
Chocolate Fudge Frosting
In a medium saucepan, combine butter and cocoa powder and bring mixture just to a boil, stirring constantly to break up cocoa powder. Remove from heat and stir in milk, powdered sugar and vanilla and mix until well combine.
If mixture is too thick to stir by hand beat it with an electric hand mixer on low speed until well blended. Immediately spread over hot cake before frosting begins to set. If adding sprinkles add right away before frosting starts to set.