Step 1: Prepare the Dough
Preheat oven to 350°F. Line baking sheets with parchment paper.
In one bowl, whisk together flour, cocoa, baking soda, and salt.
In another bowl, cream butter and sugar until fluffy.
Add egg and vanilla, beating until combined.
Add buttermilk and dry ingredients alternately, mixing until smooth.
Divide batter into two bowls. Tint one red, the other blue.

Step 2: Bake the Cookies
Use a cookie scoop to portion dough onto prepared sheets.
Leave about 2 inches between each scoop.
Bake for 8–10 minutes until the tops spring back lightly.
Cool on sheets for 2 minutes, then transfer to a cooling rack.

Step 3: Make the Filling
In a bowl, beat softened butter until creamy.
Gradually add powdered sugar.
Stir in vanilla and 1–2 tbsp milk for desired consistency.
Fold in marshmallow creme until fully combined and fluffy.

Step 4: Assemble the Whoopie Pies
Pair cookies by size.
Spread or pipe filling onto the flat side of one cookie.
Top with a second cookie to form a sandwich.
Dip edges in sprinkles or decorate with glitter if desired.