For the cake:
Preheat oven to 350°F. Line cake pans with parchment and spray with cooking spray. You'll need 2 (3×1.5-inch) round pans, 2 (6×1.5-inch) round pans and 2 (9×1.5-inch) round cake pans.
Decide what color each sized cake will be. You are making a total of 6 cakes. Use a scale to weigh the batter into 6 bowls. 6 ounces of batter per 3-inch pan, 18 ounces of batter per 6-inch pan and 41 ounces of batter per 9-inch pan. Weigh the batter accordingly into each bowl.Then add the food coloring and mix well. I did 6 ounces of yellow batter, 6 ounces of green batter, 18 ounces of blue batter, 18 ounces of purple batter, 41 ounces of red batter and 41 ounces of orange batter. Pour batter into the appropriate pans. Bake the cakes. The smallest cakes will take about 25 minutes. The 6-inch layers take about 45 minutes and the 9-inch layers take 50-60 minutes. You can test the middle to see if it’s done by inserting a toothpick. If it comes out clean, then it’s done. If not, stick it in for 5-10 minutes more.
Let the cakes cool completely. Cover and refrigerate for 2 hours or over night. They’re easier to work with that way. Place the bottom (9-inch) layer on a 12-inch cake board. Cut a 4-inch circle in the middle of the cake, letting the knife go about halfway down. Scoop out the cake from the circle. Fill to the top with M&Ms. Set aside and make your chocolate frosting. Recipe follows. Spread frosting around the top edge of the cake. Take the second layer of cake and cut another 4-inch circle in the middle, this time going all the way through the cake. Remove the middle section from that layer and save it for later. Place that layer on top of the frosted layer. Pour more M&Ms inside the hole, filling it a little more than halfway. Replace the middle section that you removed, but cut it in half horizontally so that it will fit. Frost the entire cake and make it as smooth as possible. Repeat these steps for the next 2 tiers but make the hole in the middle slightly smaller. You can use a regular dinner plate for frosting the 6-inch tier and the 3-inch tier. After frosting all 3 tiers, let them chill in the refrigerator for several hours.
Once they are chilled, place candy melts in pastry bags, one color per bag. Then place the filled bags (one at a time) in the microwave. Microwave on high for about a minute. Take it out and massage the bag gently to expedite the melting process. If it’s not completely melted, heat it a little longer. Cut a very small corner off the tip of the bag and pipe some melted candy onto the middle of the bottom (9-inch) tier. Then quickly and carefully stack your next tier on top. Use a large spatula to lift the cake up and onto the first tier. Repeat this process for the top tier, using the melted candy as your glue. Then pipe the melted candy on your frosted cake in a crazy pattern. I found that you have to move fairly quickly to get the desired effect. It’s sort of a calculated mess. Have fun with it! Let your inner artist shine through! Do as much of one color as you think looks good, then heat the next color. The candy melts will cool and harden very quickly, so you might have to reheat a little during the artistic process. Serve immediately or refrigerate until ready to serve.
For the frosting:
Melt chocolate in the top of a double boiler with gently simmering water in the lower half of the double boiler. The water should not touch the top of the pot's bottom. Stir gently to encourage melting but not vigorously. As soon as chocolate is melted, remove from heat. Allow to cool until it is just warm to the touch.
Fold in the sour cream with a large rubber spatula just until no white streaks of sour cream remain. The frosting will become overly thick with too much folding, but will remain silky if not overworked. If it becomes too stiff, you may warm it gently over a pot of warm water. It is best if used right away.