Preheat Your Oven and Grease the Pan - Preheat your oven to 350 degrees Fahrenheit. Grease the 13×9-inch cake pan with cooking spray.
Beat Cake Mix, Water, Oil, and Eggs - In a large bowl using an electric mixer, beat the cake mix, water, vegetable oil, and eggs together. Beat the mixture until smooth.
Prepare the Marble Flavors - In a small bowl, add 1/3 of the cake batter. Stir in the cocoa powder until just blended.
In the other small bowl, add in 1/3 of the cake batter. Stir in the red food coloring and the seedless strawberry jam until well combined.
Bake the Cake - Add the remaining 1/3 of the batter to the cake pan. Drop in tablespoons of the chocolate batter and strawberry batter randomly in the pan. Use a butter knife to gently swirl the batter together to create a marble effect.
Bake the cake for 28-30 minutes, or until a toothpick inserted into the middle comes out clean. Before transferring to a wire rack allow the cake to cool in the pan for 10 minutes. Let it cool for one hour before frosting.
Make the Frosting - Using an electric mixer, beat the butter in a large bowl on medium-high speed until smooth and creamy. Switch the speed to low and gradually add in the powdered sugar, beating until smooth. Beat in two tablespoons of milk and the vanilla, adding more milk if the frosting is still thick.
Color the Frosting - Add 1/3 of the frosting to a small bowl. Add the seedless strawberry jam and mix until well combined. Then, mix in a few drops of red food coloring until you reach your desired shade of pink.
In another small bowl, add 1/3 a cup of frosting. Add in the cocoa powder and mix until it is combined.
Step Seven: Frost Your Cake - Add the vanilla frosting to your fully cooled cake using an offset spatula. Then, add tablespoons of strawberry frosting and tablespoons of chocolate frosting throughout the vanilla frosting. Use the offset spatula to swirl the frosting, making a marble design.