Preheat oven to 225º F. Line two cookie sheets with parchment paper.
Place the chopped chocolate to a small, heatproof bowl that will fit snugly over a small saucepan. Set the bowl over the saucepan filled with just enough simmering water to heat the bowl without touching it. Stir the chocolate frequently until melted and smooth. Remove from heat but leave the bowl over the pan to keep the chocolate warm.
Add the egg whites and sugar to the bowl of a standing electric mixer. Whisk to combine. Set the bowl over a saucepan filled with just enough simmering water to heat the egg mixture without touching the bottom of the bowl. Stir constantly with a silicone spatula, scraping down the sides frequently until the sugar melts and the mixture reaches 140º F (once the mixture begins to lose its grittiness, check it frequently with an instant-read thermometer).
Carefully transfer the hot bowl to a mixer fitted with the whisk attachment. Whisk on medium-high speed until the meringue is light, fluffy, and just warm to the touch, about 5 minutes.
Drizzle the warm chocolate over the top of the meringue and gently fold it in with a spatula until evenly combined.
Fit a large pastry bag with a large open star tip and fill the bag with the meringue. Add a bit of the remaining meringue on the spatula or whisk to the four corners of each baking sheet underneath the parchment paper. Smooth the paper over the meringue to keep it in place during piping.
To make the cookies, begin piping in a spiral motion, moving outward from a central point. After completing about 1 1/2 rotations, begin releasing the pressure slowly as you continue dragging the tip in a circular motion. Sprinkle white nonpareils over the meringues.
Bake meringues in the preheated oven for 45 to 50 minutes, until crisp on the outside and quite soft and moist in the center. To test for doneness, lift a cookie off the baking sheet -- it should stay in one piece. When you break it in half, it should break cleanly and the center should be moist but hold its fluffy texture without collapsing.
Cool the meringues completely on the baking sheets. The centers will set up as they cool. These meringues are best fresh, but they store well in an airtight container for up to a week.