In a saucepan, thoroughly stir together the 3/4 cup sugar and cornstarch. Gradually stir in milk. Cook and stir over medium heat till thickened and bubbly. Reduce heat; cook and stir 2 minutes more. Remove from heat.
Gradually stir about 1 cup hot mixture into egg yolks. Return egg mixture to saucepan; bring to a gentle boil. Cook and stir 2 minutes more Remove from heat. Stir in butter, chopped almonds, and extract. Pour into pastry shell; cover surface with clear plastic wrap. Chill 4 to 6 hours.
In a chilled bowl, whip cream with the 1/4 cup sugar till soft peaks form. Spoon or pipe onto pie. Garnish with maraschino cherries or toasted slivered almonds. Makes 6 to 8 servings.