Light-as-air meringues are a crisp and crunchy foil for a creamy ice cream filling. Martha made this recipe on episode 507 of Martha Bakes.
Preheat oven to 300 degrees, with racks in upper and lower thirds. Line two baking sheets with parchment. Using a pencil, trace 2 1/2-inch circles, about 1 inch apart, on parchment to create templates; turn over parchment. In a small bowl, whisk together sugar and cornstarch.
In the bowl of an electric mixer fitted with the whisk attachment, whisk egg whites and salt on medium speed until frothy. Beating constantly, add sugar mixture 1 tablespoon at a time; beat until stiff, glossy peaks form, scraping down bowl halfway through, about 10 minutes. Sift cocoa and espresso powder on top and gently fold into batter in 4 or 5 additions.
Transfer batter to a large piping bag fitted with a 3/4-inch round tip. Pipe in a circular motion, about 1 inch high, onto prepared baking sheets. Bake, rotating sheets halfway through, until meringues are crisp, about 50 minutes. Let cool completely on baking sheets, about 20 minutes.
Scoop ice cream and place on the bottoms of half of the meringues; sandwich with remaining meringues. Serve immediately or cover and freeze for up to 5 days.