Add butter and, with your fingers, rub into flour mixture until well blended.
With a fork, stir in egg yolks until dough holds together. (Or whirl flour, sugar, and butter in a food processor until mixture resembles fine crumbs; add yolks and whirl until dough holds together.)
With your hands, press dough firmly into a smooth ball, kneading a bit to help bind the dough. (At this point, you may wrap the dough airtight and refrigerate up to 1 week; let come to room temperature before using.)
Press pastry into an 11-inch (28 cm) tart pan with a removable rim, pushing dough firmly into bottom and sides to make an even layer; the edge should be flush with pan rim.
Bake in a 300 F (150 C) oven, uncovered, until lightly browned, 30 to 40 minutes.
Let cool in pan. Fill tart and remove rim to serve.