1. Heat oven to 350°F.
2. Beat butter and sugar in large bowl until well blended. Add egg and vanilla; beat well. Stir together flour, cocoa, baking soda and salt; gradually add to butter mixture, beating until blended. Shape dough into 3/4-inch balls (about 1 measuring teaspoon dough). Place balls on ungreased cookie sheet.
3. Bake 8 to 10 minutes or until cookies are set (do not overbake). Cool slightly; remove cookies from sheet to wire rack. Cool completely.
4. Heat oven to 200°F. Line jelly-roll pan or cookie sheet with parchment paper. Working with a dozen cookies at a time, place half of the baked cookies on prepared pan with bottom side of cookie up. Stack two chocolate pieces on each cookie.
5. Heat cookies and chocolate 5 to 7 minutes or until chocolate is melted and can be spread with small spatula. (Do not overheat. Melted chocolate retains its shape until disturbed.) Spread melted chocolate to cover surface of cookie; top with second cookie. Allow cookies to cool until chocolate is set. Makes about 48 sandwich cookies*
* There may be extra single cookies.