There are many kinds of whoopie pies - pumpkin, red velvet, peanut butter, banana, etc. There are so many kinds that there are books with just whoopie pie recipes. But, the traditional is made with soft chocolate cookies and vanilla cream filling.
I am learning how to bake with oat flour. It is very interesting. Gluten-free baking in general (let alone "regular" baking) is kitchen science . . . much to learn. I have just touched the tip of the iceberg, so to speak. You have to learn to expand your horizon, not just follow recipes. That's when it gets fun. You'll most likely have flops somewhere along the line, but finally success comes in if you press on.
For the filling, I made a simple powdered sugar frosting: Beat together a few tablespoons lard or shortening and about 3 cups powdered sugar, adding enough water for the right consistency. Add about a teaspoon or more vanilla.
Whoopie Pies (Gluten, Dairy, Corn-free)
Adapted from Epicurious.com