Preheat the oven to 250 degrees F. Line 2 baking sheets with parchment. Place the egg whites and salt in the bowl of a stand mixer fitted with the whisk attachment (or use a hand mixer); beat on medium speed until frothy. Add the cream of tartar and increase the speed to medium high; beat in the sugar 1 tablespoon at a time, then add the extract or zest. Continue beating until the meringue is thick, glossy and holds stiff peaks, about 6 minutes.
For colored cookies, swirl a few drops of food coloring into the meringue or use a small paintbrush to paint 3 long stripes of food coloring up the inside of a pastry bag. For chocolate chip versions, gently fold the mini chocolate chips into the mixture.
Drop or pipe the meringue as desired, about 2 inches apart, on the prepared baking sheets. Sprinkle with sanding sugar, if desired. Bake 1 hour, then turn off the oven and let the meringues stand in the oven until dry, about 2 hours. Top with or dip in melted chocolate and sprinkle with nonpareils.