Preheat oven to 350°F.
Spray a 9x13 cake pan with nonstick spray.
Combine flour, granulated sugar and baking soda in mixing bowl and whisk together
In a small saucepan, heat 1 cup butter, 5 tablespoons cocoa and 1 cup cola beverage to boiling - continually stirring.
Pour over the flour mixture and mix thoroughly.
Add buttermilk, eggs and vanilla. Mixing well well.
Pour into prepared 9x13 pan.
Bake for 30 minutes.
Place 4 cups confectioners' sugar in a mixing bowl.
In a small saucepan, combine 1/2 cup butter, 3 tablespoons cocoa powder milk. Whisk together. Bring mixture to a boil.
Remove from heat
Add 1 teaspoon vanilla.
Pour over 4 cups confectioners' sugar.
Mix well with a whisk.
While cake is still warm, slowly pour frosting over the cake and spread to edges.