Heat oven to 325 degrees F. Butter two large baking sheets.
Place chocolate in microwave-safe bowl. Microwave for 1 minute, then stir until smooth. Let cool 5 minutes.
Beat butter and sugar in a large bowl until blended. Beat in cooled chocolate. Add eggs and beat until smooth. Stir in flour, salt and cocoa powder until a soft dough forms. Beat in mint extract.
Drop dough by tablespoonfuls onto prepared baking sheets. Bake at 325 degrees for 18 minutes or until dry on top. Transfer to wire rack to cool.
While cookies cool, combine white chocolate and oil in glass bowl. Microwave 30 seconds, stir; then microwave another 30 seconds. Stir until smooth. Spoon about 1 teaspoon white chocolate onto each cookie, then top each with 1/2 teaspoon of the crushed candies. Let stand to harden for 30 minutes. Store airtight up to two weeks. Makes 2-1/2 dozen cookies.