PreparationFor crusts: Combine flour and sugar in processor. Add butter and cut in, using on/off turns, until mixture resembles coarse meal. Add egg yolk and vanilla and blend just until moist clumps form. Divide dough into 10 pieces. Press 1 piece onto bottom and up sides of each of ten 3 3/4-inch-diameter tartlet pans with removable bottom. Freeze 15 minutes. (Can be prepared 1 week ahead. Cover and keep frozen.) For filling: Position rack in bottom third of oven and preheat to 350°F. Whisk first 7 ingredients in large bowl to blend. Fold in chopped pecans. Divide filling among prepared crusts. Bake until filling is firm and crusts are golden, about 30 minutes. Cool slightly. Serve warm or at room temperature.