Preheat your oven to 350°F and line two baking sheets with parchment paper.
In a large bowl, beat the butter and brown sugar for 3 minutes until light and fluffy, then beat in the vanilla.
In a separate bowl, whisk the flour, cocoa powder, baking soda, and salt.
Add the dry ingredients in three additions, alternating with the buttermilk, beginning and ending with the dry. You will get a thick, fluffy batter.
Scoop 2 tablespoon mounds onto the sheets, 2 inches apart, and bake for 10-12 minutes until the tops spring back when touched.
Cool on the sheets for 5 minutes, then transfer to a rack to cool completely.
For the filling, beat the butter for 2 minutes, add the powdered sugar on low, then add the cream, vanilla, and salt and whip for 3 minutes until pale and fluffy.
Spread or pipe the filling onto the flat side of half the cookies, top with the rest, and press gently to sandwich.
