Secret to flat cakes: wet cake strips (i., Bake Even Strips) and wrap around the cake pans. They act as an insulator to help the cake bake evenly (flat). You can also use strips of towel: drench, wrap around the pans, and bake.
These Gender Reveal Party Cupcakes have pink, blue, and white swirled frosting on top that hides the secret colored heart in the center of the cupcake revealing the baby's gender when you bite into it! (Step-by-Step Photo Tutorial)