Preheat the oven to 350°F. Line two large baking sheets with parchment paper. Set aside.
Whisk the flour, cocoa powder, baking soda, and salt together.
In a large bowl, beat the butter for 1 minute on medium-high speed until completely smooth and creamy. Add the brown sugar and beat on medium high speed until fluffy and combined. Beat in egg on high speed, scraping down the sides and bottom of the bowl as needed. Once egg is full combined, beat in the vanilla and buttermilk.
On low speed, slowly mix the dry ingredients into the wet ingredients until combined. Add enough food coloring to achieve a deep red hue (usually about 1 teaspoon gel). Spoon or scoop mounds of batter, about 1 Tablespoons each, onto prepared baking sheets– about 3 inches apart.
Bake for 10-12 minutes. Allow to cool completely before sandwiching.
Cream Cheese Filling:
Beat the cream cheese for 1 minute on high speed until smooth. Beat in butter until combined. Add the powdered sugar and vanilla and beat on medium-high speed until creamy and combined.
Pair the whoopie pies based on their size. Spread filling onto the flat side and sandwich together. Repeat with the remaining whoopie pies.