1. Preheat oven to 200 degrees. Line rimmed baking sheets with parchment paper; set aside. Divide meringue among three small bowls. Set one aside. Fold 1 tablespoon cocoa into the second portion, and 2 tablespoons cocoa into the third portion. Place each portion in an 18-inch pastry bag fitted with a large round 1/2-inch tip. Pipe domes, 1/2 inch to 2 inches in diameter, onto prepared baking sheets, and flatten the tips with a damp forefinger. Pipe stems onto baking sheets, making one to go with each "cap."

2. Bake the meringue shapes in the upper and lower thirds of the oven, rotating baking sheets halfway through, 1 hour; reduce oven temperature to 175 degrees. Continue baking until completely dry to the touch but not browned, 45 to 60 minutes.

3. Melt bittersweet chocolate in a small heatproof bowl over a pan of simmering water, stirring occasionally. Using a small offset spatula, spread bottoms of caps with a thin layer of dark chocolate, and let set. Melt white chocolate in another heatproof bowl set over a pan of simmering water. Let cool until thickened, and spread over dark chocolate. Use a toothpick to draw lines through the chocolate from the center to edge of caps; let set.

4. Using a paring knife, make a small hole in center of each coated cap. Dip one end of each stem in remaining white chocolate, and insert into a hole; let set.