Pulse together flour, sugar, salt & powder in bowl of food processor. With processor running, add butter until mix is crumbly. Add yolk & pulse 2-4 times. With machine running add water until dough comes together. Remove dough from processor & gather into a ball. Flatten into a disk. Wrap in plastic & chill 30 minutes. Roll out on lightly floured board to 10" circle. Ease into 7" tart pan with removable bottom. Remove excess dough. Chill 1 hour.