I've spent the majority of my life in New England and we claim the title of home of the whoopie pie (or gobs as they're known elsewhere). While their true origins may not be 100% clear, as a New Englander there are a few things I hold true when it comes to whoopie pies:
1. Real whoopie pie filling is not frosting. If I want a mouthful of cake frosting, I'll eat a cake.
2. Real whoopie pie filling is made with marshmallow Fluff and vegetable shortening. It may sound a little gross to use shortening, but it creates the fluffiest filling.
3. Whoopie pie shells are not cake. They're cake-like, for sure, but they are not cake and trying to use boxed cake for shells will not work out the way you hope. I speak from experience :) Whoopie pie shell batter is a bit thicker and fluffier.
Step 1: BoM
3-3 1/2 cups cake flour
1 1/2 cups sugar (you can add an extra 1/2 cup of sugar if you want a sweeter shell)
1/2 cup butter
1/2-3/4 cup vegetable shortening
2 tsp baking soda
2 tsp salt
2-3 tsp vanilla extract
1 cup milk + 1 tbs vinegar (may look like it's curdling, it's fine...but maybe do this last)
Mini chocolate chips
3-4 cups of powdered sugar
1-1 1/2 cups of vegetable shortening
6-10 tbs milk
1 small jar of marshmallow fluff
4 tbs vanilla extract
mini chocolate chips (optional)
Additional extracts for different flavors can be added, maple, cocoa powder, etc. are great additions.
Step 2: Filling
If you don't have a stand mixer, you can do make the filling at any point. You want the filling to mix for 20-30 minutes, or longer if you can, the goal is fluffiness.
Add the fluff, shortening, vanilla extract, milk, powdered sugar, and flour to a bowl and mix for as long as you can. Stop and taste test every so often and make adjustments as needed.
Step 3: Mix
Preheat oven to 350.
Mix the dry ingredients in one bowl and the wet (not the milk + vinegar) in another, then slowly add the dry ingredients to the wet while mixing. Alternate adding dry ingredients and milk until everything is well combined. Stir in chocolate chips.
Step 4: Bake
I used an ice cream scoop to scoop out smallish balls and placed the shells 2-3 inches apart. Cook them for about 10 minutes and then remove to a wire rack to cool.
Try to resist the urge to leave the shells in the oven for a few more minutes.
Step 5: Fill
When your shells are completely cool you can start layering them with filling and sandwich them together. For an added touch you can roll the whoopie pies in chocolate chips, nuts, etc.
Step 6: Enjoy
Your whoopie pies will keep in the fridge for a few days or the freezer for a few weeks (I love "frozen" whoopie pies). You'll want to wrap each pie individually in plastic wrap for best results.