HEAT oven to 350°F Place pecans, 1/2 cup powdered sugar and salt in bowl of food processor fitted with metal blade. Pulse machine on and off to break up nuts, then process until nuts are finely ground. Add butter a few pieces at a time, pulsing on and off to incorporate, then process until mixture is smooth; pulse in vanilla. Add flour and pulse machine on and off until incorporated, scraping the dough down once or twice. Process until the dough begins to form a ball.
REMOVE dough from machine, form into a very flat disc, wrap in plastic wrap and refrigerate at least 2 hours or until firm enough to roll into balls. Dough may be refrigerated overnight.**.
LINE two rimmed jelly-roll pans with parchment paper. Roll dough between lightly floured palms into 1-inch balls and place on prepared cookie sheets 2-inches apart. Make an indentation with your thumb or finger in center of each cookie to make a deep well, going about three-quarters of the way down into the cookie. If cracks form, simply press any cracks together.
BAKE 13 to 15 minutes or until light golden brown on the bottoms and around the edges. Place cookie sheets on cooling racks; sift remaining 1 1/2 cups powdered sugar over warm cookies. Immediately fill the indentations with jelly and allow to cool completely.
*To toast pecans: Place pecans in dry nonstick skillet; cook over medium heat, shaking pan until nuts are lightly browned.
**You may freeze dough up to 1 month; defrost in refrigerator overnight before proceeding.
Cookies may be stored at room temperature in an airtight container for up to 2 weeks; these are best stored in single layers separated by waxed or parchment paper.