1. Make cookies: Preheat oven to 350 degrees. Line 2 baking sheets with parchment paper. In a medium bowl, whisk flour, cocoa powder, baking soda, and salt; set aside.

2. With an electric mixer, cream butter and sugar until light. Add egg and vanilla; beat until incorporated. Reduce speed to low, and gradually mix in flour mixture (dough will be soft). Mix in chocolate chips by hand.

3. Drop mounds of dough (each equal to 2 level tablespoons) onto baking sheets, spacing as far apart as possible. With fingers, make mounds as round as possible (you should have 14 to 15).

4. Bake, switching baking sheets from top to bottom halfway through, until edges of cookies are firm but not hard, 15 to 20 minutes. Cool 1 minute on baking sheets; transfer cookies to racks to cool completely.

5. Assemble sandwiches: Scoop 1/3 cup ice cream onto flat side of each of 6 cookies. Top each with another cookie, and press gently to form a sandwich (you have extra cookies, in case of breakage).

6. Freeze sandwiches until firm, at least 1 to 2 hours, before serving (or up to a week, wrapped individually in plastic wrap).