Peel, halve and core pears. Brush with lemon juice. Place pears cut side down in pie shell with narrow ends toward center.
Cream together butter and sugar. Beat in flour, eggs, vanilla and salt. Pour over pears. Sprinkle lightly with mace.
Bake in moderate oven (350°) 45 minutes or until filling is set and lightly browned. Cool on rack 1 hour or longer before cutting. Top with whipped cream. Serve the same day you bake the pie—pear filling may darken if you keep pie overnight.