Heat oven to 350°F. Line two baking sheets with parchment.
Whisk flour, cocoa, baking powder, baking soda, and salt in a bowl.
Cream softened butter with granulated and brown sugars until light and fluffy.
Beat in egg, vanilla, and oil. Mix in red coloring until evenly tinted.
Add dry mix in thirds, alternating with buttermilk. Mix just until smooth.
Scoop heaping tablespoons of batter 2 inches apart. Smooth any peaks.
Bake 10–12 minutes until set and springy. Rotate pans halfway through.
Cool on sheets 5 minutes, then transfer cookies to a rack to cool completely.
Beat cream cheese and butter for the filling until creamy and smooth.
Gradually mix in confectioners' sugar, vanilla, salt, and cream. Whip until fluffy.
Pair cookies by size. Pipe or spread filling on half, then cap with the rest.
Chill assembled whoopie pies 15 minutes to set before serving.