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.

Frosting:

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.