Combine sugar, flour and salt. Add milk slowly, mixing thoroughly. Cook over low heat until thickened, stirring constantly. Stir in a small amount of hot mixture into egg yolks, then pour back into remaining hot mixture while stirring vigorously. Cook 3 minutes longer. Remove from heat and add butter and vanilla. Cool. Slice bananas into pie shell; cover immediately with filling. Top with sweetened whipped cream or use the 2 egg whites to make a meringue.