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.
Whoopie Pies began as a New England tradition and were associated with Amish farmers. They consist of two large flat cakes with a creamy filling. But Whoopie Pies have now become one of the hottest new gourmet trends with mixes, pans and whoopee extras being sold in high end gourmet stores.