Fit piecrust in 11-inch tart pan.* Sprinkle chocolate and walnuts evenly over crust.
Microwave butter in large glass measure, covered, 40 seconds. Beat in brown sugar, corn syrup, eggs and vanilla until smooth. Pour into pan.
Bake in lower third of 350°F oven 30 minutes. Cool on rack. Serve with whipped cream if desired.
Tart can be made in 9-inch pie pan; bake 45 minutes.