Preheat oven to 350°F and grease a round 9 inch cake pan or a square casserole dish.
In a mixing bowl, beat together mayonnaise, sugar and vanilla until incorporated.
In another bowl, whisk together flour, cocoa powder, baking soda and salt until combined and lump free.
Mix half of the dry ingredients into the mayonnaise mixture. Then stir in the coffee. Add the remaining dry ingredients and mix until just incorporated. Pour into prepared baking dish.
Bake for 25-30 minutes or until the cake springs back when lightly touched with your finger.
Cool in pan for 10 minutes, the turn out onto a wire rack if desired. (This cake is also great cut and served from the baking dish.)
Small Batch Chocolate Frosting
Beat the butter until smooth.
Add the remaining ingredients and beat well.
Spread over cooled cake and decorate with sprinkles if desired.