My first post with the new T1i camera!
Have you ever made something and just know that it doesn't look right? Well in my case it didn't smell right! As I followed the recipe and added the buttermilk, it didn't smell right at all. I realized that in trying to use up ingredients, I grabbed the wrong buttermilk and it was expired, really expired. I know that buttermilk naturally smells a bit more sour but this was wrong, so wrong. For the first time in baking, I started over from scratch.
Now that you are grossed out, I have to say that once completed these were delicious! The chocolate cake cookies were a soft and puffy perfection, they were crumbly like a cake, but solid like a cookie. The icing is the creamiest I have made and really tasted like marshmallow fluff.

Chocolate Marshmallow Whoopie Pies
Cookies:
3 and 1/2 cup all-purpose flour
1 and 1/2 cup unsweetened cocoa powder
1 tsp salt
1 tbsp baking soda
1 tsp baking powder
1 cup unsalted butter, softened
2 cups sugar
2 large eggs
2 cups buttermilk, room temperature
2 tsp vanilla extract
Filling:
1 cup unsalted butter, softened
2 cups confectioners sugar
7 ounces marshmallow fluff
2 tsp vanilla extract
Preheat oven to 400 degrees. Line 2 baking sheets with parchment paper or baking mats. Sift together flour, cocoa powder, salt, baking soda, and baking powder, set aside. In stand mixer blend butter and sugar till fluffy, add in eggs one at a time. Beat in vanilla and buttermilk till incorporated. Slowly add in the dry ingredients only till combined. Using a level cookie scoop drop batter on baking sheet and bake for 12 minutes. Cool for 5-10 minutes on baking sheet and cool completely on drying rack.
For filling, beat butter on medium till smooth. Mix in confectioners sugar till combined. Add in vanilla and marshmallow fluff and beat on medium-high for five minutes or until fluffy. Fit pastry bag with large round tip (I used Wilton #12) and fill with icing. After cookies are cooled completely, find some soul-mates, squeeze icing onto one side, and sandwich them together until icing is pushed to the edges.

I also didn't follow Annie's directions by putting two pans in the oven at once, I thought it would bake more evenly if I only put in one pan like normal. But when making cake, you know how you are supposed to mix it right before you put it in the oven, well I think the same goes for this recipe. The last two batches, that sat out for at least 30 minutes, went into the oven and came out flat, so I do recommend using two pans at a time and rotating them halfway through baking. You were right Annie.
I did love this recipe and like the idea of other whoopie pies in the future, what other combos have you made?