Preheat oven to 350ºF. Coat two baking sheets with cooking spray or line with parchement paper and set aside.
In a large mixing bowl, use an electric mixer on medium speed to mix butter and sugar until well blended, about 3 minutes. Add eggs, one at a time, mixing until smooth after each egg. Mix in vanilla extract.
In a separate bowl, stir together flour, baking soda, and salt. Add half the flour mixture to the butter mixture and mix on medium speed until combined. Add half the buttermilk and mix again on medium speed until smooth and slightly fluffy in texture. Repeat with remaining flour and buttermilk and mix until smooth. Batter will be thick and slightly springy.
Drop 2 tbsp portions of batter onto prepared baking sheets, leaviing 2 inches between each portion to allow for spreading. Bake for 11-13 minutes, until puffed and set, but still soft when touch lightly with fingertops. Let cakes cool for 3 minutes on baking sheets before tansferring them to wire racks to finish cooling, about 15-20 minute Repeat with remaining batter.
Prepare filling while cakes are baking and cooling. In a medium bowl, sift together cocoa powder and confectioner's sugar and set aside. In a large mixing bpwl, use an electric mixture on medium speed to cream butter. Add milk, vanilla extract, salt, and half the cocoa mixture. Mix on low speed firstt o combine, then on high until smooth, Add remaining cocoa mixture and mix again. Scrape down sides of bowl, add marshmallow creme, and mix on medium-high until filling is smooth and fluffy, about 3-4 minutes.
Assemble whoopie pies. Spoon filling onto the flat sides of half the cakes, dividing it evenly among cakes. Top with remaining cakes, flat side against the filling, rounded side up. Serve immediately, or wrap each whoopie pie individually in plastic wrap and store at room temperature for up to two days, or in freezer for up to two months.