- 2 large egg whites, at room temperature
- 1 teaspoon pure vanilla extract
- 1 teaspoon instant espresso powder
- ¼ teaspoon kosher salt
- 1 pinch cream of tartar
- ½ cup sugar
- 4 ounces bittersweet chocolate, melted and cooled to room temperature
- Heat oven to 200° F.
- Using an electric mixer, beat the egg whites with the vanilla, espresso powder, salt, and cream of tartar on medium-high speed until soft peaks form. Very gradually (1 tablespoon at a time) beat in the sugar; increase mixer speed to high and beat until the mixture holds glossy peaks.
- Drop heaping tablespoonfuls of the mixture onto parchment-lined baking sheets, spacing them 1 inch apart.
- Bake until dry, crisp, and firm, about 2 hours. (The meringues are done when they release easily from the parchment.)
- Turn off the oven, prop open the door about ½ inch (use the handle of a wooden spoon), and let the meringues sit inside for 1 hour. Slide the parchment (with the meringues on top) onto racks and let cool completely.
- Peel the meringues off the parchment, then dip the bottom of each into the chocolate, allowing any excess to drip off. Place on parchment-lined baking sheets and refrigerate just until the chocolate is set, 25 to 30 minutes. Store the cookies in an airtight container at room temperature for up to 3 days.