Stir together flour, cocoa powder, salt, and baking soda in a small bowl, and set aside. In a mixing bowl, beat butter and sugars until creamy. Add espresso powder and vanilla and beat until blended. Stir in egg whites. Add the dry ingredients and beat to incorporate, then knead with your hands until you have a smooth dough.
Form the dough into 2 logs, about 2 inches in diameter and 14 inches long. Wrap each log in wax paper and chill at least 1 hour or up to three days.
Preheat oven to 350. Slice chilled logs into 1/4" thick cookies. Bake on heavy sheets lined with parchment or silpat, about 12 minutes (longer for crisper cookies, shorter for chewy cookies). Cool on rack.
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