I did a 1/4 sheet cake that used two boxes of cake mix for this Blues Clues cake. Mine was half chocolate and half white. I had my husband help pour the batter so we did it at the same time and then I marbled it in the middle.
When I got ready to start doing the frosting I was preoccupied with the kids and forgot to put it in the freezer first. I did one thin layer that ended up full of crumbs and then froze it for about an hour and put another thin layer of white on to hide those imperfections. I then put it back in the freezer while I prepared the pattern.
To do the pattern I used my editing software to blow up the pattern on two pages and taped them together. Then I cut it out and gently laid it on top of the cake and traced all the details with a toothpick. I filled in the details with the star tip and even pressure. Add googly eyes to finish.