How to Make It
Prepare the Cake: Preheat oven to 375°F. Grease and flour a 13- x 9-inch cake pan; line bottom of pan with parchment paper, and lightly grease with cooking spray.
Sift together sugar and flour in a large heatproof bowl; set aside. Combine butter, water, and cocoa in a medium saucepan over medium-high. Bring to a boil, stirring constantly, until butter melts. Remove from heat, and pour over sugar mixture, stirring until dissolved. Cool slightly, about 15 minutes. Stir in buttermilk, eggs, vanilla, baking soda, and cinnamon until combined. Pour into prepared pan. Bake in preheated oven until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Cool in pan on a wire rack for 20 minutes. Invert on a cooling rack, and cool completely, about 1 hour.
Prepare the Chocolate-Cream Cheese Frosting: Beat together cream cheese and butter with an electric mixer on medium speed until creamy. Beat in melted chocolate until combined. Gradually add powdered sugar, beating until light and fluffy. Beat in vanilla extract. Spread top and sides of Cake with Chocolate-Cream Cheese Frosting. Garnish with sprinkles, if desired.