Heat oven to 350°F. Fit piecrust into a 9-inch removable-bottom tart pan. Sprinkle chocolate and walnuts evenly over crust. Set aside.
Microwave butter in large glass bowl, covered, 40 seconds. Whisk in espresso powder. Beat in brown sugar, corn syrup, eggs and vanilla. Pour into crust.
Bake in lower third of oven at 350°F for 37 to 40 minutes. Cool on rack to room temperature. Cut into slices; serve with whipped cream, if desired.