Stir together first 4 ingredients; firmly press on bottom and sides of a 9-inch pieplate.
Bake at 350° for 10 to 12 minutes or until lightly browned. Remove to a wire rack, and let cool.
Whisk egg yolks just until blended; whisk in lime juice. Add milk, whisking until blended. Stir in lime rind. Pour into prepared crust.
Bake at 350° for 12 to 15 minutes or until set. Remove from oven; cool completely on a wire rack. Cover and chill 8 hours.
Beat cream until foamy; gradually add sugar, beating until soft peaks form. Gently stir in vanilla, and, if desired, rum. Spoon whipped cream on top of pie; garnish, if desired.