Fit piecrust into a 10" tart pan with a removable bottom according to package directions. Line pastry with aluminum foil, and fill with dried beans. Bake at 450° for 7 minutes. Remove dried beans and foil, and bake piecrust 2 more minutes; cool on a wire rack. Reduce oven temperature to 350°.
Beat butter and brown sugar in a large bowl at medium speed with an electric mixer until fluffy; add eggs, 1 at a time, beating well after each addition. Stir in flour and buttermilk. Add vanilla, stirring well. Pour filling into prepared crust.
Bake at 350° for 45 minutes or until almost set. Cool completely on wire rack. Remove tart pan rim and transfer tart to a serving platter. Dust with powdered sugar, if desired. Dollop each serving with whipped cream.
If you're going to top the tart with whipped cream, keep in mind that cream doubles when it's whipped. For 2 cups of whipped cream, buy a half-pint (1 cup) carton of whipping cream. Use a deep metal bowl, and place the bowl, the beaters, and the cream in the freezer for 15 minutes before whipping to get the best volume.