Add brown sugar, baking soda and salt. Beat until combined, scraping sides of bowl occasionally. Beat in the sour cream, egg, melted chocolate and vanilla until combined. Stir in flour until just combined. Drop dough by slightly rounded teaspoonfuls about 3-inches apart onto an ungreased cookie sheet.
Transfer cookies to a wire rack to cool completely.
Frost cooled cookies with Chocolate Butter Frosting. 3 tablespoons milk 1 teaspoon vanilla extract In a medium mixing bowl beat butter until fluffy. Gradually add 1 cup of the powdered sugar and the cocoa powder, beating well. Gradually beat in the milk and vanilla. If necessary, beat in additional milk to make frosting a spreadable consistency.