Heat oven to 375°F. In food processor, place flour, 3/4 cup of the powdered sugar and 1/2 teaspoon salt. Cover; process until well blended. Add butter and 1 1/2 teaspoons of the vanilla; process with on-and-off pulses until mixture is consistency of fine crumbs.
Press 1 1/2 cups of the mixture evenly in ungreased 9-inch tart pan with removable bottom. Bake 12 minutes. Remove to cooking rack; cool 15 minutes.
Meanwhile, in small bowl, stir preserves to soften; set aside. In another small bowl, mix mascarpone cheese, remaining 1/4 cup powdered sugar and remaining 1/2 teaspoon vanilla until well blended. Cover; refrigerate.
Spread preserves evenly over cooled crust to within 3/8-inch of edge. Sprinkle remaining crust mixture evenly over top, covering preserves completely. Pat gently; sprinkle with white sparkling sugar.
Bake 18 to 26 minutes longer or until edges are golden brown. Cool 25 minutes on cooling rack; remove side of pan. Using sharp knife, cut into 12 wedges. Serve warm with sweetened mascarpone mixture.