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.
When ready, preheat the oven to 400°F. Line 2 baking sheets with parchment paper.
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.