• Preheat oven to 400°F and lightly grease a ½ sheet jelly roll pan.
• In a large bowl, combine flour and sugar. Set aside.
• In a saucepan combine and bring to a boil: butter, unsweetened cocoa powder and water. Once it boils, remove from heat. 
• Pour chocolate mixture over flour and sugar mixture and mix well with a wooden spoon.
• Add the buttermilk, eggs, baking soda, salt, cinnamon and vanilla. Stir until smooth.
• Pour batter into prepared pan and bake for 20 minutes or until the center springs back. Do not over bake.

Icing

• Sift the powdered sugar into a medium bowl being sure to get rid of any lumps. Set aside.
• In a small saucepan, combine and boil; butter, unsweetened cocoa powder and milk, stirring often. 
• Remove from heat and pour over sifted powdered sugar. Add vanilla and stir until smooth.
• Mix well and pour onto cooled cake and spread out evenly. Top with sprinkles, mini chocolate chips or chopped nuts.