1 Let refrigerated piecrust stand at room temperature according to package directions.
2 Ease piecrust into an 11-inch tart pan with removable bottom. Trim piecrust even with the rim of the pan. Do not prick piecrust.
3 For filling, in a large mixing bowl beat eggs slightly with a rotary beater or fork. Stir in the corn syrup. Add the brown sugar, butter, and vanilla, stirring until sugar is dissolved. Stir in the nuts and the 1/2 cup chocolate pieces.
4 Place pastry-lined tart pan on baking sheet on oven rack. Carefully pour filling into pan. Bake in a 350° oven for 40 minutes or until a knife inserted near the center comes out clean. Cool on a wire rack.
5 To serve, cut tart into wedges and transfer to dessert plates. In a small, heavy saucepan cook and stir the 1/3 cup chocolate pieces and the shortening over very low heat until chocolate begins to melt. Immediately remove from heat and stir until smooth. Cool slightly. Transfer chocolate mixture to a clean, small, heavy plastic bag. Snip a very small hole in 1 corner of the bag. Drizzle the melted chocolate in zigzag lines across each tart wedge.
6 If desired, serve tart with vanilla ice cream. Cover and refrigerate any leftover tart for up to 2 days.