Line a 16 x 4 x 4.5-inch loaf pan with non-stick aluminum foil or grease with shortening and dust with flour.
Make cake batter using two cake mixes, according to package instructions.
Divide the batter in 5 bowls. Color each bowl with different colours of your choice.
Pour the batter into disposable pastry bags or zip top bags.
Drizzle colored cake batter, alternating colors into the pan until you use all the batter.
Tap the pan on the counter a few times to remove air bubbles.
Bake for 70-80 minutes until a toothpick inserted in the center of the cake comes out clean.
Let cake cool in pan for about 20 minutes, then remove and set cake on cooling rack and cool completely.
Cut cake into 1-inch thick slices. Lay slices flat on a baking tray and place in the freezer for at least one hour.
Cut the cake slices with your chosen cookie cutter - whatever letter or number you want to have in the middle of your cake. Place the letter/number pieces back in the freezer. You can save the left overs for other purposes.
Clean and prepare the loaf pan as before.
Make the remaining two cake mixes according to package instructions.
Pour a thin layer of cake mix into the loaf pan.
Pour remaining batter into an 18-inch pastry bag or a gallon-size zip top bag.
Line the letters/numbers in a single row down the center of the pan, piping batter into the pan as needed to keep the pieces straight.
Pipe the remaining batter into the pan and on top of the letters/numbers. Be sure to mark which direction the numbers are facing and keep track of this while decorating, so you know which end of the cake to slice into and have forward-facing design.
Smooth the batter.
Bake for 70-90 minutes, rotating pan 180 degrees after 40 minutes, until a skewer or cake tester inserted into the center of the cake comes out clean.
Let cake cool completely.
Color fondant in a variety of neon colors.
Roll individual pieces of the colored fondant into logs. Press the logs together. Twist and fold to create a marble effect. Make sure not to overdo it or your colours will look muddy. Press fondant pieces together. Roll out to 24x15-inches.
Cover cake with fondant.
Create a border using a different color fondant.
Serve the cake and slice into it, revealing the design in the middle.
Here are a few tips in handling fondant:
Keep it covered. Fondant dries out quickly, so it's best to keep it in a zip top bag or wrapped in plastic wrap.
Wear food-handling gloves. No matter how clean your hands are it will always get marks which can be visible on white fondant.
Wear gloves when coloring the fondant too, to prevent stains.
Don't use too much cornstarch. It will dry out your fondant.
Use a fondant that tastes good to you. There are many brands of fondant on the market, so find one you really like.