1. Preheat oven to 350°.
2. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, baking soda, and salt in a medium bowl, stirring with a whisk.
3. Place brown sugar and butter in a medium bowl. Beat with a mixer at medium speed until creamy; gradually add granulated sugar, beating well. Add cocoa and egg whites, beating well. Gradually add flour mixture, beating until well blended.
4. Drop by rounded teaspoonfuls 1 1/2 inches apart onto baking sheets coated with cooking spray.
5. Bake at 350° for 10 minutes or just until edges are set. Cool on pans 2 minutes; remove from pans. Cool completely on wire racks.
6. Spread about 2 tablespoons ice cream onto flat side of 18 cookies; top each with another cookie, pressing gently to form a sandwich. Freeze until firm, about 2 hours.