Preheat the oven to 200°F. Line two half sheet pans with parchment paper.

In the bowl of a stand mixer fitted with the whisk attachment, whip the egg whites and cream of tartar on medium speed until frothy. Then, with the mixer running, stream in the sugar and whip until white in color and beginning to grow in volume, but not yet able to hold soft peaks.

Meanwhile, in a small saucepan, whisk the cornstarch with ⅓ cup water. Cook over medium- high heat, stirring continuously, until thickened, about 2 minutes.

With the mixer running, slowly pour the hot cornstarch slurry into the egg white mixture, followed by the salt and vanilla. Raise the mixer speed to medium-high and whip until the mixture holds stiff peaks, 10 to 12 minutes. Gently fold in the chocolate and walnuts.

Spoon 2-tablespoon mounds of the meringue mixture onto the prepared sheet pans, spacing them 1 inch apart. Bake for 2 hours, then remove from the oven and let cool completely.

Serve once cool, or store in an airtight container lined with paper towels at room temperature for up to 4 days.