Process flour and pecans in food processor until pecans are very finely chopped. In medium bowl, beat butter and sugars until light and fluffy. Stir in flour mixture. Wrap dough in plastic wrap and refrigerate overnight.
Preheat oven to 300°. On lightly floured surface, use floured rolling pin to roll out dough to 1/4-inch thickness. Use thimble to cut out cookies. Transfer to foil-lined baking sheet. Bake 10 to 12 minutes or until very lightly browned. Makes about 50 dozen cookies.
Variation: Cookies may be rolled out as directed above and cut out using desired cookie cutters. Bake 20 to 25 minutes. Makes about 5 dozen 2 1/2-inch cookies.