Coarsely chop the almonds in a food processor or with a knife. Set aside.
Whisk together the cocoa powder, confectioner's sugar, and salt in the bowl of an electric mixer. Combine the vanilla and egg whites in a measuring cup or bowl, and slowly add to the cocoa mixture, beating at low speed. Increase to medium speed and beat until mixture is glossy, about 2 minutes. Stir in the chopped almonds.
Drop 1/4 cup mounds of batter, 3 inches apart, onto a large baking sheet lined with parchment paper. Immediately reduce the oven temperature to 325°F. Bake until small thin cracks appear in the tops of the cookies and they are nearly firm to the touch, about 16 minutes. Cool cookies on the parchment paper, on a cooling rack, invert, and peel carefully from the paper.
Return the oven to 350°F. and repeat the process with the remaining batter.
When cookies are completely cool, wrap tightly, separated by parchment paper. These freeze well for 3-6 weeks.
Makes 10 large cookies.