• Preheat oven to 350 degrees F. 
• Line an 18×13″ rimmed baking sheet with parchment and spray with nonstick baking spray

CAKE

1. Melt 1 cup of butter in a pan over medium low heat.
2. Once melted, in the same pan, add 1 cup of water and 4TBSP of cocoa until just mixture just starts to boil. Transfer to the boil of a standing mixer. 
3. Add  2 cups of sugar and mix on medium for 20 seconds. 
4. Add 1/2 cup of sour cream and mix until incorporated. 
5. Add two eggs and vanilla and mix until incorporated.
6. Add 2 cups flour, 1 tsp baking soda and cinnamon and mix for 10 seconds on medium speed just until incorporated.
7. Transfer with a spatula onto the prepared rimmed baking pan. Batter will be quite thin.
8. Bake in a preheated oven for about 15-20 minutes. Because of its thinness, it bakes fast, so be sure to check it at 15 minutes with a cake tester or knife to ensure that it is baked through. Let it cool.

FROSTING:

Once the cake is in the oven, start preparing the frosting. 

1. Melt 1/2 cup of unsalted butter in a clean small saucepan on medium low heat.
2. Add 4 TBSP of unsweetened cocoa powder and 6 TBSP of milk to the saucepan and stir with a whisk just until it starts to come to a boil.
3. Pour warm mixture into a clean standing mixer bowl. Add 3 1/2 cups of powdered sugar, I tsp vanilla and 1/2 tsp cinnamon and mix until frosting comes together into a creamy cohesive texture. 

Frost the cake while still warm with a large angled spatula.  The frosting will melt into the cake and look smooth and glossy versus creamy when it is done. If you would like to add toasted nuts, sprinkles or mini chocolate chips, add them right at this point. As the frosting begins to set, toppings will no longer stick to the surface. 

Let the cake cool until ready to serve and then pre-slice it for easy access and distribution.