Preheat oven to 350 degrees. Beat together butter, sugar and brown sugar. Add eggs and vanilla. In a separate bowl combine flour, baking soda and salt, then add dry ingredients to dough. Stir in chocolate chips. Roll into small, ½ inch balls and place on cookie sheet. Bake for approximately 10-12 minutes. Remove from oven and let cool for 5 minutes on cookie sheet, then place on wire rack to cool completely.
While cookies are baking and cooling start making the filling. Cream together butter and brown sugar. Add powdered sugar in increments until smooth. Add in the flour and salt, then add the milk and vanilla.
Once cookies have cooled begin making the sandwiches. Place a spoonful (as much or as little as you want) on the flat side of one cookie, and sandwich it with the flat side of another cookie. Repeat until all cookies are gone. Coat sides with mini chocolate chips if desired.