Preheat oven to 350°F (175°C).
In a mixing bowl, cream together butter and sugar until light and fluffy.
Add vanilla extract and mix well.
In another bowl, whisk together flour, cocoa powder, baking soda, and salt.
Gradually add dry ingredients to the butter mixture, mixing until combined.
Roll dough into small balls and place on a baking sheet.
Flatten each ball slightly with the bottom of a glass.
Bake for 8-10 minutes, or until cookies are set.
Allow cookies to cool completely.
For the filling, beat together butter, powdered sugar, and vanilla extract until smooth.
Spread filling on the flat side of one cookie and top with another cookie.