Directions

1. Preheat oven to 375 degrees. Butter a 10-by-15-inch rimmed baking sheet; dust with flour, tapping out excess. Set aside. In a bowl, whisk together flour, cocoa, baking powder, and salt.
2. Cream butter and sugar until smooth. Add eggs and vanilla; beat until light and fluffy. With mixer on low, gradually beat in flour mixture. Pat dough into a rectangle. Wrap in plastic; refrigerate 30 minutes.
3. Roll out dough between wax paper into a 10-by-15-inch rectangle. Remove top paper; use the bottom to flip dough onto prepared sheet. Score into 20 squares with a paring knife. Pierce holes over squares.
4. Bake until just firm, 8 to 10 minutes, rotating sheet halfway through. Cut into squares; let cool completely.
5. Sandwich ice cream between two cookies. Wrap in plastic; freeze to set, 5 to 10 minutes. Serve plain, or place on plates, and drizzle with hot fudge.