TO MAKE THE CAKE:
Heat oven to 375°F. Grease a 9x13 or 10x15 inch rimmed sheet pan.
Whisk the flour, sugar, brown sugar, baking soda, salt, and cinnamon together in a large mixing bowl.
In a separate bowl, whisk the buttermilk, vanilla, and eggs together.
In a small saucepan over medium heat, whisk the butter, cocoa powder, and water together just until the butter is melted and cocoa powder is dissolved. Remove from heat.
With the mixer on low, slowly drizzle the cocoa mixture into the dry ingredients and continue to blend until well combined.
Slowly pour in the wet ingredients and continue to mix until well combined. Scrape down the bowl as needed.
Spread the cake batter evenly in a greased sheet pan.
Bake for 15-17 minutes or until toothpick inserted in center comes out clean. Remove from oven and set on a cooling rack. Immediately make the icing.
TO MAKE THE ICING:
Melt the butter in a small saucepan over medium-low heat. Whisk in the cocoa powder, milk, and vanilla. Slowly incorporate the powdered sugar, whisking until combined.
By this time, the cake should have cooled for about 5-10 minutes but still be warm. Pour the icing over the cake and work quickly to spread it evenly. Cool completely before serving.
For a larger cake, the recipe can be doubled using a 13 X 18-inch or 12 x 17-inch pan.