Preheat the oven to 180C and line 2 large baking sheets with parchment paper. In a medium bowl, whisk the flour with the cocoa powder, baking soda, baking powder and salt. In a large bowl, using a handheld electric mixer, beat the butter with the brown sugar at medium speed until fluffy, about 2 minutes. Beat in the egg and vanilla. At low speed, beat in the dry ingredients and buttermilk substitute in 3 alternating additions. Scrape down the side and bottom of the bowl as necessary.
Using a 3-tablespoon ice cream scoop, scoop 12 level mounds onto each baking sheet, about 2,5cm to 4cm apart. Bake the cakes in the lower and upper thirds of the oven for about 15 minutes, until risen and firm. Then, shift the pans from top to bottom and front to back halfway through baking. Transfer the sheets to racks and let the cakes cool completely.
In a large bowl, beat the butter with the confectioners' sugar and vanilla seeds until fluffy. Beat in the marshmallow fluff.
Divide the filling into three bowls and mix in one, the orange coloring, in the other the green one and the last, leave it white. Then add in three different pastry bags fitted with a large plain tip.
Pipe the three colors of filling onto the flat sides of 12 of the cakes and close the whoopie pies, pressing to evenly distribute the filling to the edges. Roll the edges.
Cut the sugar paste and the black sugar paste and assembly to create 24 eyes, then add them on the whoopie pies and serve.