To make the sandwich cookies:Add the flour, baking powder, baking soda, cocoa powder, and salt to a bowl and whisk to combine. Set aside and line two sheet pans with parchment paper and preheat the oven to 375℉.

In the bowl of a stand mixer, cream the butter and sugar for 2 minutes until light and fluffy on medium speed.

Add the egg and vanilla on medium-low speed and mix until just combined (15-20 seconds)

Scrape down the sides of the bowl with a rubber spatula.

With the mixer running at low speed, alternate adding the dry ingredients and the milk, starting and ending with the dry ingredients.

Scoop the batter, 2 tablespoons in size onto the prepared sheet pans. Space the cookies out 2” away from each other to accommodate spreading. 

Bake for 10-11 minutes until risen in the center and a toothpick inserted comes out clean. Also, when you poke the cake with your finger, the whoopie pie should spring back.

Let the whoopie cookies cool completely before filling.

Make The Filling: Beat the butter on medium low speed for 30 seconds until smooth.

Add the powdered sugar, milk, vanilla, and salt.  Mix on low speed until just combined. Turn up the speed to medium and whip until light and fluffy for 5 minutes. 

Add the fluff, and beat 1 minute on medium high speed to incorporate, the frosting will be thick.

To assemble the whoopie pies, flip half of the cooled whoopie cookies over and scoop two tablespoons of filling onto each half. Spread the filling out to edges. Top with the remaining whoopie cookies and serve.