Make the cookies first. Preheat oven to 325 F. Line a baking sheet with parchment paper and set aside.
In a small bowl, whisk together the flour, cocoa powder and salt.
In the bowl of an electric stand mixer, combine the butter and brown sugar until light and fluffy. Stir in the vanilla. Slowly add the flour mixture and beat until just combined. It will still be a bit crumbly.
Turn the mixture out onto a clean work surface, and knead it a few times to bring it all together. Dust the work surface with cocoa or powdered sugar. Working swiftly and carefully, roll out the dough and cut out 2-inch circles with a biscuit cutter. Transfer to a baking sheet, spacing them about an inch apart (a small offset spatula makes this step easier). Re-roll the scraps and cut out the remaining circles.
Bake 20 minutes, or until the tops are no longer shiny. Let cool completely.
To make the filling, beat the butter with an electric mixer until creamy. Gradually mix in the powdered sugar. Add the extracts and salt, and beat until combined.
To assemble, fill a piping bag with the filling and pipe a small amount into the center of half of the cookies. Top with remaining cookies to make little sandwiches. Refrigerate for a few minutes before serving. Store uneaten cookies in the refrigerator.