Preheat oven to 350 degrees F. Line a large baking sheet with parchment paper and set aside. Invert 8 shot glasses or tea cups on the counter.
In a medium bowl, whisk together the egg whites, sugar, and pistachios until just combined. Add the butter and vanilla, and whisk. Add the flour and cocoa powder, and whisk.
Drop the batter by 1 1/2 tablespoonfuls onto the baking sheet, about 4 inches apart. Using a small spatula or the back of a spoon, spread the batter out to form a thin 4 1/2 -inch circle (the batter needs to be a thin, even coat because the cookies will not spread when cooked). Bake for 12 to 13 minutes, or until a deep brown. Remove from the oven.
With a thin spatula, quickly remove the cookies 1 at a time from the baking sheet and drape on top of a ramekin to create a cup shape. (If the cookies become too brittle, replace on the baking sheet and place in the oven for 10 seconds to soften.) Cool completely on the glasses, then gently lift off and transfer to a platter until ready to use.
Yield: 8 cookies