• 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 bowl 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. 
9. While the cake is baking, prepare the frosting so it is ready to pour as soon as the cake leaves the oven.

FROSTING:

1. Start the Frosting: Once the cake is in the oven, start preparing the frosting. Melt 1/2​ cup of unsalted butter in a clean small saucepan over medium-low heat.
2. Bring to a Boil: Add 4 tablespoons of unsweetened cocoa powder and 6 tablespoons of milk to the saucepan. Stir with a whisk just until the mixture starts to come to a boil, then remove from heat.
3. Mix Until Creamy: Pour the warm mixture into a clean stand mixer bowl. Add 3.5 cups of powdered sugar, 1 teaspoon of vanilla, and 21​ teaspoon of cinnamon. Mix until the frosting comes together into a creamy, cohesive texture.
4. Frost While Warm: Pour and spread the frosting over the cake while it is still warm using a large angled spatula. The frosting will melt into the cake and look smooth and glossy versus creamy when it is done.
5. Add Toppings Immediately: 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.
6. Cool and Slice: Let the cake cool completely until the frosting is set. Pre-slice it before serving for easy access and distribution at your potluck!