Preheat oven to 350 and line baking pans with parchment paper.

In the bowl of a stand mixer, or with a hand mixer and large bowl, cream butter and brown sugar until light and fluffy, about 2-3 minutes.  Add the eggs and mix to combine.  Mix in the flour, cocoa, baking soda, and salt.  Stir in the buttermilk and mix until well incorporated.

Scoop equal portions of dough onto the baking pans spacing the dough balls 2-3 inches apart.  Bake for 10-12 minutes until the tops spring back when touched.  These are like cakes, not brownies so we want them cooked through.  Allow the cookies to cool fully while you mix the filling.

In a stand mixer or with a hand mixer and medium bowl, mix the butter, powdered sugar, vanilla, and salt until light and fluffy.  Add the marshmallow creme and mix to combine.

Spread or pipe filling onto half of the cookies, and top with the remaining cookies to form a sandwich,  If desired, pour sprinkles into a shallow bowl or pie pan and roll the sides of the whoopie pies through the sprinkles.