Preheat oven to 200°F (95°C). Line a baking sheet pan with parchment and trace 10, 2 1/2-inch (6 cm) circles onto paper. Flip paper over.

In a clean, grease-free bowl whip egg whites with balloon whisk attachment of stand mixer on low speed until frothy. Add cream of tartar and continue beating, turning speed to high, until soft peaks form. Add sugar gradually and beat until meringue is stiff and glossy. (If using a hand-held mixer, simply beat longer and use visual cues).

Scoop into a pastry bag fitted with a large open star tip such as a Wilton 1M or 4B. Starting in the center of a traced circle, start piping concentric circles until the outline is filled. Then, along the outside edge of the circle, pipe a ring above the first, making a sidewall of meringue. Alternatively, simple scoop out with 2 spoons, creating nests and use spoons to make a well in the center of each nest.

Bake for about 2 hours or until completely dry and crisp to the touch, rotating pan front to back once during baking. Cool pan on wire rack. Store shells in a dry place for up to a week.