In large bowl, beat butter, sugar, eggs and vanilla until light and fluffy. Stir together flour, cocoa and salt; gradually add to butter mixture, beating until well blended.
Refrigerate dough about 1 hour or until firm enough to handle.
Heat oven to 350°F. Shape dough into 1-1/8-inch balls; roll in extra sugar. Place on ungreased cookie sheet. Bake 8 to 10 minutes or until set. Remove from oven.
Place a Hershey kiss on each cookie, pressing down lightly. Cool completely.