To avoid creating water pockets in your brisee (which will wreak havoc in your dough), make sure to strain the ice out of the water before drizzling it in and processing.
Pulse flour, salt, and sugar in a food processor until combined. Add butter and pulse until mixture resembles coarse meal with some pea-size pieces remaining. Drizzle 5 tablespoons water over mixture; pulse several times to combine. Add more water,1 tablespoon at a time, and pulse until mixture holds together when pinched.
For a 9-inch pie, shape dough into two disks and wrap each in plastic. For a slab pie, shape dough into a rectangle and wrap in plastic. Refrigerate at least 1 hour and up to 1 day, or freeze up to 3 months; thaw overnight in refrigerator before using.
Be sure to use cold, not frozen, butter when making pate brisee. Frozen butter won't incorporate as easily, and you'll end up having to use more water than is ideal.