Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
In a small bowl, combine dry ingredients: flour, cocoa powder, salt, and baking soda.
In a large mixing bowl, beat together butter, granulated sugar, and brown sugar until combined and fluffy (because of the large proportion of brown sugar, the butter and sugars will not cream into a fluffy, pale yellow mixture).
Beat eggs into butter/sugar mixture until batter is fluffy. Add vanilla extract and combine.
Add in red gel or liquid food coloring and beat, scraping down the sides of the bowl thoroughly to ensure the color is evenly distributed throughout the batter. Add enough food coloring to make a vibrant/dark red.
Beginning and ending with the buttermilk, rotate adding the buttermilk and sifted dry ingredients until all ingredients are incorporated. The red color of the batter will deepen with the addition of the cocoa powder.
Use a 1 ½ or 2 Tablespoon cookie scoop and place even scoops of batter onto the prepared pans. Leave about 2” of space between scoops.
Bake for about 10 minutes or until the cake springs back when pressed gently with your finger.
Remove the cakes from the oven and cool on the cookie sheets for 3-4 minutes before transferring to a cooling rack to cool the rest of the way. While cakes are cooling, prepare the cream cheese filling.
Beat together softened cream cheese, butter, and vanilla extract until combined.
Beat in 3 cups of powdered sugar.
Add 1 tablespoon of heavy cream at a time until the desired consistency of frosting is achieved.
Use the same size cookie scoop that you used to form batter scoops to add the proper amount of frosting into each whoopie pie.
Place a scoop of frosting onto the center of the flat side of one of the cakes. Sandwich the frosting together with a second cake and squeeze gently until frosting reaches the edge of the whoopie pie.
Serve to store in an airtight container in the refrigerator until ready to serve.
You can achieve a brighter red color rather than a deeper red by decreasing the amount of cocoa powder to 4 tablespoons, but the flavor is especially good with the recipe as written.