Special equipment: a 3-tablespoon (#20) cookie scoop, a metal teaspoon and a pastry brush
Preheat the oven to 250 degrees F. Line a baking sheet with parchment.
Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment; beat on medium speed until frothy. Add the cream of tartar and increase the speed to medium high. Beat in the granulated sugar 1 tablespoon at a time, adding a couple drops of black food coloring with the final tablespoon of sugar to tint the mixture light gray. Beat in the vanilla if using. Continue beating until the meringue is thick and glossy and holds stiff peaks, 6 minutes more.
Spray a 3-tablespoon (#20) cookie scoop with nonstick spray and scoop balls of the meringue onto the prepared baking sheet, about 1 inch apart. Lightly dust the tops with cocoa powder--you should see meringue underneath.
Bake the meringues until set, about 1 hour. Remove from the oven, turn off the oven and keep the oven door closed. Let the meringues sit until cool enough to handle, about 5 minutes. Then, hollow them out this way: Hold a meringue flat-side up in one hand. With the edge of a metal teaspoon, gently carve a hole in the center of the meringue's base, leaving some lip around the opening. Press your thumb into the hole and push the inside of the meringue up against the walls to hollow out the interior (see Cook's Note).
Return the meringues to the baking sheet and place them in the turned-off oven until completely dried out, at least 3 hours and up to 8 hours.
With a wooden spoon, smash the rock candy sticks in their bags to break the sugar off the sticks and into smaller crystals. Pour the crystals into small bowls separated by color.
Place the white candy melting wafers in a microwave-safe bowl and microwave in 30 second intervals, stirring between each, until melted.
Use a pastry brush to paint a circle of melted wafers (about the size of a quarter) on the bottom of a meringue cavity. Immediately fill the cavity with dark-colored rock candy. Wait a few seconds, then pour the excess rock candy back into its bowl. Paint the rest of the cavity with melted wafers. Immediately fill the cavity with light-colored rock candy. Wait a few seconds before pouring out the excess. Repeat with the remaining meringues, wafers and rock candy.
This recipe is inspired by Kingston Candy Bar, Kingston, NY. If a meringue breaks, simply glue it back together with white chocolate. Save the meringue crumbs for sprinkling over ice cream, lattes and hot chocolate.
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