Melt chocolate in top of double boiler over hot, not boiling, water.
Remove from heat; cool.
Cream butter and brown sugar in large bowl.
Beat in vanilla, egg, melted chocolate and baking soda until light and fluffy.
Stir in flour to make a stiff dough.
Divide dough into 4 parts.
Shape each part into a roll, about 1 1/2 inches in diameter.
Wrap in plastic wrap; refrigerate at least 1 hour or up to 2 weeks.
Preheat oven to 375°F.
Line cookie sheets with parchment paper or leave ungreased.
Cut rolls into 1/8 inch-thick slices; place 2 inches apart on cookie sheets.
Bake 6 to 7 minutes or until firm.
Remove to wire racks to cool.
Prepare Creamy Mint Filling.
Spread filling on bottoms of half the cookies.
Top with remaining cookies, bottom sides down, forming sandwiches.