In a large bowl, beat 1/2 cup butter or margarine until creamy. Add the sugar, and beat until mixed well. Beat in egg and mint extract.
Sift flour, cocoa, and salt together into a small bowl. Add flour mixture by halves into creamed mixture, beating well after each addition.
Divide dough in half. On lightly floured surface roll dough into two 1 1/2 inch diameter cylinders. Wrap each cylinder in waxed paper, and refrigerate 5 hours.
Preheat oven to 350 degrees F (175 degrees C). Thirty minutes prior to baking, place both cylinders in freezer.
Remove one cylinder at a time, and slice 1/4 inch thick pieces with very sharp knife. Place on cookie sheets about 1 1/2 inches apart. Bake 10 to 12 minutes.
Melt 1/4 cup butter or margarine and the semisweet chocolate in a double boiler or in the microwave. Drizzle over warm cookies. Place on wire racks, and let cool and harden completely.