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.