1. In a food processor, finely grind 1/2 cup pecans with the sugar. Add flour and butter; process until moist clumps form. With wet fingers, press crust evenly into bottom and up sides of a 9-inch pie plate. Freeze until firm, about 20 minutes. Preheat oven to 375 degrees.

2. With a fork, prick bottom of crust all over. Bake until golden, 20 to 25 minutes (if crust puffs up, gently press down with the back of a spoon). Let cool completely.

3. Spread ice cream evenly in cooled crust. Chop remaining 1/2 cup pecans; sprinkle on top. Freeze, covered loosely with plastic wrap, until firm, at least 6 hours and up to overnight.