Preheat the oven to 400 degrees. Line a 11x17-inch sheet pan with parchment paper and spray the edges with non-stick cooking spray.
In a large mixing bowl, combine the flour, sugar, baking soda, and cinnamon. Whisk to blend.
In a medium saucepan place 2 sticks (1 cup) butter, water and 4 tablespoons cocoa powder. Bring to a boil, stirring.
Pour the hot cocoa mixture into the bowl with the flour mixture, and blend well with a hand mixer. Add the eggs, buttermilk, and vanilla extract. Beat well. The batter will be thin. Pour the batter into the prepared pan. Bake 10 to 12 minutes or until cooked through. Place on a rack in the pan.
Meanwhile, in the same sauce pan, melt the remaining stick (1/2 cup) butter. Add the remaining 4 tablespoon cocoa powder and the milk. Bring just to a boil. Pour into a mixing bowl with the almond extract and powdered sugar. Beat with a hand mixer until smooth. Keep beating until the cake comes out of the oven.
Immediately spread the hot icing on the hot cake. The icing will harden quickly. Sprinkle the almonds over the icing and allow it to set.