From Cook's Illustrated. These were much easier than I anticipated and totally delicious. Because of the nature of the batter, you do have to bake the cookies as soon as possible, so don't double the batch unless you have double the oven space too. (In a related note, thank you to upstairs neighbors for emergency oven use!)
3/4 cup sugar
2 teaspoons corn starch
2 teaspoons instant espresso powder
4 large egg whites
3/4 teaspoon vanilla extract
1/8 teaspoon table salt
1. Adjust two oven racks; one to the upper middle position, one to the lower middle position. Heat oven to 225°. Line two baking sheets with parchment paper.
2. Combine sugar, cornstarch and espresso powder in a small bowl and set aside. In a stand mixer fitted with the whisk attachment, beat egg whites, vanilla and salt on high speed until very soft peaks begin to form. Reduce speed to medium and slowly add the sugar in a steady stream down the side of the bowl. Stop mixer and scrape down the sides of the bowl with a rubber spatula. Return mixer to high speed and beat until glossy, stiff peaks have formed.
3. Working quickly, place meringue in a pastry bag fitted with a 1/2" plain tip, or a ziploc bag with one tip snipped off. Pipe merigues onto cookie sheets in 1 1/4" wide mounds, 6 rows of 4 meringues on each sheet. Bake for 1 hour, rotating sheets halfway through baking. Turn off oven and allow meringues to cool in oven to 1 hour. Remove from oven and let cool. Store in an airtight container for up to two weeks.
I added bittersweet mini chocolate chips to mine, which made them even better!