Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Coat 2 baking sheets with cooking spray. Whisk the flour, cocoa powder, baking soda and salt in a medium bowl; set aside.
Put the butter and 2 ounces of the chopped chocolate in a large microwave-safe bowl; microwave in 30-second intervals, stirring, until smooth. Whisk in the sugar, egg and vanilla until smooth. Stir the flour mixture into the chocolate mixture until smooth.
Gently roll tablespoonfuls of dough into balls; arrange about 1 1/2 inches apart on the prepared baking sheets. Gently flatten the cookies with your fingers. Bake, switching the pans halfway through, until the edges of the cookies are just set, about 12 minutes. Immediately press a marshmallow half, cut-side down, into the center of each cookie. Let cool 3 minutes on the baking sheets, then transfer to racks to cool completely.
Microwave the remaining 6 ounces chocolate in a medium microwave-safe bowl in 30-second intervals, stirring, until smooth; stir in the coconut oil. Let stand until slightly thickened, about 10 minutes. Spoon the chocolate over the marshmallows, then use the back of a small spoon to gently spread the chocolate so it covers the cookies. Decorate with nonpareils. Let set, 1 hour.