Whisk together the flour, cocoa, baking soda, baking powder and salt in a large bowl, set aside.

Beat the granulated sugar and butter together in another large bowl with an electric mixer until light and fluffy, about 3 minutes.

Add the egg and vanilla and mix until incorporated.

Alternate adding the dry mixture and the milk to the egg and vanilla mixture, beginning and ending with the dry mixture, mix until just incorporated. Refrigerate the dough until chilled, at least 30 minutes and up to overnight if covered.

Drop tablespoonsful of batter about 2 inches apart on the prepared baking sheets.

Bake until the whoopie pies puff up and the tops are no longer glossy, 7-8 minutes. Let cool for a few minutes on the baking sheets, then transfer to a wire rack to cool completely.

Add heavy cream to mixing bowl, mix starting with low speed, then gradually increase to medium speed until the cream starts getting thicker (the gradual speed progression prevents you from spraying yourself with cream).

The cream will thicken and start to peak. Stop the mixer and add in the peppermint extract. Mix on high speed until the cream is thick and stiff.