Preheat oven to 375°F and line two baking sheets with parchment paper.
In a food processor or stand mixer, combine flour, cocoa powder, baking soda, baking powder, vanilla and sugar.
With the mixer on low (or pulsing in a food processor), add the butter pieces gradually, then mix in the egg and vanilla extract until the dough comes together.
Scoop 1-teaspoon-sized balls of dough onto the baking sheets, spacing them 1 inch apart. Lightly flatten each ball to about ⅛-inch thickness. Or roll them out with a cute emboss rolling pin.
Chill for 10 minutes.
Bake for 7-9 minutes, rotating the trays once. The cookies should be set and fragrant. Let them cool completely on the baking sheets.
Make the Cream Filling: In a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until light and fluffy, about 3 minutes.
Add the vanilla and salt and beat to combine.
Reduce the speed to low and gradually add the powdered sugar, beating until fully incorporated.
Increase the speed to medium-high and beat until the filling is light and fluffy, about 2 minutes more.
Spread or pipe the filling onto the flat side of half the cookies, then top with the remaining cookies to make sandwiches.
