Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a medium bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.

In a separate large bowl, cream together butter and sugar until light and fluffy.

Add the egg and vanilla extract to the butter mixture and beat until combined.

Gradually add the dry ingredients to the wet ingredients, alternating with buttermilk. Mix until fully incorporated.

Drop tablespoon-sized dollops of batter onto the prepared baking sheet, spacing them about 2 inches apart.

Bake for 10-12 minutes or until tops are set. Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

While the cakes cool, prepare the filling. In a bowl, beat butter, powdered sugar, marshmallow fluff, and vanilla extract until smooth and fluffy.

Divide the filling into separate bowls and tint with pastel food coloring as desired.

Pipe or spread the filling onto the flat side of half of the cookies, then top with the remaining halves to create sandwiches.

Decorate the edges with festive sprinkles for an Easter touch.