Heat oven to 450°F. Place pie crust in 10-inch tart pan with removable bottom as directed on box for One-Crust Filled Pie. Carefully line crust with a double thickness of foil, gently pressing foil to bottom and side of crust. Bake 7 minutes. Carefully remove foil; bake 2 minutes longer. Cool on cooling rack 10 minutes. Reduce oven temperature to 350°F.
In medium bowl, beat butter and brown sugar with electric mixer on medium speed until fluffy. Add eggs, one at a time, beating well after each addition. Stir in flour and buttermilk. Stir in vanilla. Pour filling into partially baked crust.
Bake 45 minutes or until almost set. Cool completely on cooling rack. Remove tart from pan; place on serving plate. Sprinkle with powdered sugar. Serve with whipped cream.