Heat the egg whites and sugar in the top of a double boiler set over barely simmering water until warm to the touch (this will help you get more air into the whites when you whip them). Transfer to a mixer fitted with a whisk attachment and whip until soft peaks form. Add the mint extract and continue whipping just until stiff and glossy. Fold in the chopped unsweetened chocolate.
Scrape the mixture into a pastry bag. Pipe bite-size kisses onto the cookie sheets and bake until the meringues are the color of milky coffee, 25 to 30 minutes. To test, remove one meringue from the oven, let cool one minute, then taste. It should be dry and crisp all the way through. Let cool on the pans.
Melt the semisweet chocolate. Dipping the tines of a fork into the chocolate, drizzle the meringues with melted chocolate. Let sit until chocolate is set, 30 minutes to an hour (or let set in the refrigerator for 15 minutes).