Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a mixing bowl, cream together the butter and sugar until light and fluffy. Add the vanilla extract and mix well.
In another bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the butter mixture, mixing until combined.
Roll the dough out on a lightly floured surface to about ¼ inch thickness. Use a cookie cutter to cut out circles and place them on the prepared baking sheet.
Bake for 8-10 minutes, or until the cookies are set. Allow to cool completely on a wire rack.
For the filling, beat the butter until creamy. Gradually add the powdered sugar and vanilla extract, beating until smooth.
Spread the filling onto half of the cookies and top with the remaining cookies to form sandwiches.