gridiron sheet cake

Chocolate Sheet Cake:
1 cup water
1 cup butter
1/4 cup unsweetened cocoa powder
2 cups all-purpose flour
2 cups white sugar
1/2 teaspoon salt
2 eggs
1/2 cup sour cream
1 teaspoon baking soda
Buttercream Frosting:
1/2 cup butter, softened
1/2 cup shortening
1 teaspoon vanilla extract
4 cups confectioners' sugar
2 tablespoons milk
Decoration:
2 tablespoons cocoa powder
3 drops green food coloring
piping bags and tips
wooden toothpicks or skewers
6 chocolate-covered caramel candies (such as Rolo®)
4 pretzel sticks
1 (5 ounce) bag gummy bears
1 cup boiling water
1 (12 ounce) package white chocolate chips
1 (12 ounce) bag dark chocolate chips

Preheat the oven to 375 degrees F (190 degrees C). Grease a jelly roll pan.
Bring water, butter, and cocoa powder to a boil in a large saucepan. Remove from heat and stir in flour, white sugar, and salt.
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.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 22 minutes. Let cake cool completely.
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.
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.
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.
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.
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.
Place cake in the freezer for 1 hour. In the meantime, make your decorations.
Stack 3 caramel candies and glue them together with chocolate frosting. Snap 1 pretzel stick in half and use chocolate frosting to glue one half to each end of a full stick. Adhere to the top of the caramels with more frosting. Repeat to make another goalpost.
Sort gummy bears into 2 sets of team colors.
Fill a mug with boiling water and place offset spatula inside. Remove the cake from the freezer. Thoroughly dry the spatula and use it to smooth any lumpy frosting on the cake. Repeat if the spatula cools down.
Carefully plot out chalk lines for the field using a ruler and toothpick or skewer. Draw 6 dotted lines 1 inch apart. Carefully cover all of the dotted lines with white chocolate chips.
Line each end zone with dark chocolate chips to create a border with the field, leaving room for the goalposts in the middle. Arrange gummy bears on the field.