In saucepan, melt butter over medium-low heat. Stir in sugar and flour. Remove from heat; set aside. In mixing bowl beat eggs with electric mixer on medium speed just until fluffy (about 1 minute). Stir in buttermilk and vanilla. Gradually whisk buttermilk mixture into butter mixture until smooth. Pour into prebaked crust.
Place pie on oven rack; carefully tent whole pie with foil. Bake 50 to 55 minutes or until center is set when gently shaken. Cool on wire rack for 1 hour. Cover; refrigerate at least 4 hours before serving. Makes 8 servings.