In a small bowl, whisk together the flour, cocoa powder, espresso powder, baking powder, baking soda, and salt and set aside.
In a separate bowl whisk together the melted butter, granulated sugar, and brown sugar until fully combined. Then add in the egg and mix until smooth.
Add in half the flour mixture and mix until combined, then add in the milk and the remaining flour mixture and whisk until the batter is smooth.
Print out 2 football templates and place next to each other on the baking sheet. Top with a sheet of parchment paper and secure with magnets.
Scoop the batter into a large piping bag fitted with a medium round tip.
Pipe a football shape outline onto the parchment paper (about 3 inches long), then fill in with batter. Repeat with the remaining batter, spacing the footballs about 2 inches apart.
If using the templates, remove the magnets and gently slide them out from underneath the parchment paper after you're done piping the footballs.
Bake for 8-10 minutes or until the cakes are set and firm to the touch. Bake one sheet at a time on the center rack for the best results.
While the first set is baking, pipe the second set of footballs on a separate baking sheet.
Remove from the oven and let cool on the baking sheet for about 5 minutes. Then gently peel them off the parchment paper and let cool on a wire rack.
Marshmallow Fluff Buttercream
While the cakes are cooling, make the filling.
In the bowl of a stand mixer fitted with the paddle attachment (or with a large bowl and hand mixer), cream together the softened butter, powdered sugar, and salt until smooth and creamy.
Add in the marshmallow fluff and vanilla and mix until combined. Then turn the mixer on high speed and beat until the filling is light and fluffy. It should be thick but still spreadable.
If the frosting is too thick, add in 1-2 teaspoons of milk to help thin it out.
Assembly
Once the football cakes are fully cooled, turn half over and spread with a few tablespoons over the frosting. Then top with another football to make a sandwich.
Spoon about ¼ cup of the frosting into a piping bag fitted with a small round tip (or small sandwich bag with the corner cut off). Then pipe lines on top to make the whoopie pies look like footballs.
Notes
Store the football whoopie pies in an airtight container lined with parchment paper, in the fridge for up to 4 days. For best results, do not stack the whoopie pies!! They are very moist, so it's best to store them in a single layer so they don't stick together! Let come to room temperature for about 30 minute before serving for the best taste/texture.