Instructions

Combine flour, baking soda, and salt in medium bowl. Melt butter and sugar in medium saucepan over low heat. Add chocolate, cover pan with lid, and let stand off heat until chocolate melts, about 4 minutes. Stir until smooth. Transfer chocolate mixture to bowl of stand mixer fitted with paddle attachment and let cool, about 10 minutes.
With mixer on medium speed, beat egg into cooled chocolate mixture until thoroughly combined. Reduce speed to low and mix in flour mixture until just combined, about 1 minute. Cover dough with plastic wrap and refrigerate until firm, about 1 hour.
Meanwhile, adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper.
Roll 2 teaspoons dough into 1-inch balls and place 2 inches apart on prepared sheets. Bake until cookies are just set, 10 to 12 minutes, rotating and switching sheets halfway through baking. Remove from oven, top each hot cookie with 1 mint, and let chocolate soften, about 1 minute. Working with one at a time, using knife, spread softened mint over top of cookie and transfer to rack to cool completely, about 30 minutes. (Cookies can be stored in airtight container at room temperature for up to 1 week.)