Preheat oven to 350F degrees. Spray a 9x9-inch square pan with nonstick baking spray.
In a large bowl, mix together all the dry ingredients (flour, sugar, baking cocoa and baking soda)
In a separate bowl, whisk together water, mayonnaise, vanilla extract and coffee. You just want to use plain, brewed coffee - no additional milk or sugar added.
Slowly add the liquid mixture into the dry ingredients.
Stir just until combined.
Pour into prepared baking dish.
Bake for about 30-35 minutes (until a toothpick inserted comes out clean).
Once the cake is finished, start preparing the frosting. In a medium saucepan, over medium heat, combine brown sugar and butter and cook until bubbly. Be sure to whisk constantly so it doesn't burn.
Take off the heat and stir in the milk.
Then gradually add in powdered sugar. Whisk until smooth.
Frost the cake!
Video
Notes
The cake doesn't have to be cooled completely to top with this frosting. Work quickly. The frosting hardens fast as it cools.