• Preheat the oven to 350°.
• In the bowl of a stand mixer fitted with the paddle attachment, add the sugar and butter. Beat that for 2 minutes until light and fluffy.
• Add the eggs and coffee, continue to mix until everything is combined.
• In a separate bowl, mix the flour, cocoa powder, baking powder, baking soda, and salt together.
• Add the dry ingredients to the wet ingredients. Turn the stand mixer on low and mix everything together until combined. Add the milk to loosen up the batter. Continue mixing until a thick batter is formed.
• Prepare two baking sheets by placing parchment paper or silicone baking sheets on them. With an ice cream scoop, scoop the batter on to the baking sheets. Be sure to leave about an inch between each as they grow during baking.
• Place the baking sheets in the oven for 15 minutes. When the pies are done, take them off the baking sheets one by one and place them on a wire rack to cool. Continue baking until all of the batter is done.

To make the buttercream

• Place the butter in a large bowl. Using a hand mixer or a stand mixer fitted with the whisk attachment, beat the butter for 2 minutes on high until it is light and fluffy. Add 4 cups of powdered sugar, milk, and vanilla. Continue to beat the mixture, starting at low speed so you don't get powdered sugar everywhere. Beat the mixture until a thick buttercream forms. If the buttercream is too thick, add more milk, one tablespoon at a time. If the mixture is too thin, add more powdered sugar, one tablespoon at a time.

Putting the Whoopie Pies together

• When the pies are completely cooled, place them all so the flat side is facing up.
• Place the buttercream into a piping bag.
• Pipe a circle of buttercream around the edge of half of the pies. If you have 12 large pies, only pipe circles on 6 of them so you have a bottom with the buttercream, and a top to place on top of the buttercream.
• Add a teaspoon of sprinkles to the middle of the whoopie pie. When the buttercream and sprinkles are added to the pie, top with another pie that has no buttercream on it.
• Serve immediately.