Spray a 9x13 baking pan with cooking spray. Preheat the oven to 350ºF.
For the cake, put flour, sugar, cocoa, baking powder, baking soda, cinnamon, cayenne pepper (optional) and salt in a large bowl and mix well.
In a stand mixer, mix the buttermilk, oil, eggs and vanilla together. Slowly add in the mix of dry ingredients while the mixer is on low. Add the coffee and make sure everything is combined. The batter will be very thin.
Pour batter into the prepared pan and bake for about 30-35 minutes or until the center of the cake springs back when you touch it. You can also insert a toothpick into the center and it should be mostly clean.
Allow the cake to cool on a cooling rack. While the cake is cooling, make the frosting.
Make the frosting:
Whip the butter alone in a stand mixer on medium for 2 minutes or until fluffy, almost white in appearance and lighter in texture.
Add the vanilla and cinnamon. Add the powdered sugar slowly, mixing between each addition.
Alternate the last bit of powdered sugar and the heavy cream, and whip the frosting on high for at least 2 minutes. If the buttercream is too thick after 5 tablespoons of heavy cream, add another tablespoon.
Spread the frosting over the cooled cake. Sprinkle the top with mini chocolate chips.
Store in an airtight container at room temperature.
Refer to the article above for more tips and tricks.The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**