Preheat oven to 325 degrees. Line baking sheet with parchment paper. Combine first 3 ingredients using a food processor. Cut in butter until the mixture resembles coarse meal. In a small bowl, beat the yolks with the vanilla extract and add to the flour mixture. Beat until completely blended. Pinch off pieces of dough and roll into balls. Place on the lined cookie sheet. Make indentation with thumb, fill the hole with preserves and press on a pecan half. Bake for 20 to 25 minutes.