Step 1
Preheat the oven to 375 degrees F (190 degrees C). Grease a jelly roll pan.
Step 2
Bring water butter and cocoa powder to a boil in a large saucepan. Remove from heat and stir in flour white sugar and salt.
Step 3
Beat eggs sour cream and baking soda in a bowl; stir into flour mixture until just blended. Pour batter into the prepared jelly roll pan.
Step 4
Bake in the preheated oven until a toothpick inserted into the center comes out clean 20 to 22 minutes. Let cake cool completely.
Step 5
Make frosting while cake is cooling. Cream butter shortening and vanilla extract together in a large bowl. Blend in confectioners' sugar 1 cup at a time beating well after each addition. Add milk; continue beating until frosting is light and fluffy.
Step 6
Divide frosting into 3 bowls. Mix cocoa powder into the first bowl until it's dark and chocolaty. Mix green food coloring into the second bowl. Set third bowl aside. Fill 3 piping bags with the chocolate green and plain frosting; attach round tips.
Step 7
Use a ruler to measure out 2 equal 'end zones' on each side of the cooled cake. Use a skewer or toothpick to poke a few holes to mark where the end zones will be.
Step 8
Pipe a big green square 'playing field' between the end zones. Cover the entire square with green frosting; smooth out using a small offset spatula.
Step 9
Create the end zones. Pipe a rectangle on one side of the field using chocolate frosting. Fill the rectangle with frosting; smooth out with a clean offset spatula. Repeat on the other side of the field using the plain buttercream.
Step 9
Place cake in the freezer for 1 hour. In the meantime make your decorations.