STEP 1. Cut out a 3-inch round circle from the center of 3 of the layers. (You should have one layer remaining that is intact with no hole cut out.)
STEP 2. I used my first cut out as a guide to show me where to make my cut into the next layer.
STEP 3. Using the first batch of white frosting, frost the three, donut-hole shaped cake layers together. Make sure to save enough frosting to frost the top of the fourth layer and the sides of all four layers (as shown in step 5.)
STEP 4. Fill the hole all of the way up with M&Ms! I also put 4-5 toothpicks throughout my cake at this point to stabilize the layers. I kept them away from the front so when they cut into it, the toothpicks wouldn't show.
STEP 5. Place the fourth, whole layer and frost all sides with the remaining white frosting. It's OK if the sides are a bit messy, that will all be covered up with pretty frosting. Depending on your design for the top, however, you may want to make it a little smoother and thicker. Also, I put one more toothpick once the final layer was on top.
STEP 6. Now for the fun! Make the second batch of frosting, split it in two and color half pink and half blue. I used a 12 round tip for the piping and it worked great! Starting from the back, pipe 6 circles in PINK in a straight line.
STEP 7. Using an icing spatula, spread each circle of icing to the right. (I had two icing spatulas so I could alternate and not make a mess) Also, have some paper towels handy so you can wipe any excess icing onto them.
STEP 8. Once you have spread each circle, repeat step 6 by piping 6 circles in BLUE slightly on top of the spreads of pink and then repeat step 7. Do this all around the cake until done!
STEP 9. For the top of their cake I placed a huge question mark?... but there are so many cute ideas out there! Best of luck and have fun!