For the Whoopie Pies:

1. Preheat the oven to 350°F (180°C) and line two baking sheets with parchment paper.
2. Mix dry ingredients: In a large bowl, sift together the flour, cocoa powder, baking soda, and salt. Set aside.
3. Cream butter and sugar: In another bowl, cream the softened butter and brown sugar until light and fluffy (about 2 minutes). Add the egg and vanilla extract, and beat until well combined.
4. Alternate wet and dry ingredients: Gradually add the dry ingredients and buttermilk to the butter mixture, alternating between the two. Start and end with the dry ingredients, mixing until just combined.
5. Scoop and bake: Using a spoon or cookie scoop, drop the batter onto the prepared baking sheets, leaving about 2 inches between each one. Bake for 11-13 minutes or until the tops spring back when touched. Let them cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.

For the Filling:

1. Make the filling: In a mixing bowl, beat the softened butter until smooth. Add the powdered sugar a little at a time, mixing on low speed. Once combined, add the heavy cream and vanilla extract, then beat on high for 3-4 minutes until light and fluffy.
2. Assemble the whoopie pies: Pair up the whoopie pies by size. Pipe or spread the filling onto the flat side of one whoopie pie and gently press a second one on top to create a sandwich.
3. Serve and enjoy: Serve immediately or store in the fridge for later. Bring to room temperature before serving.