Preheat oven to 350°. Stir together first 2 ingredients; add butter, stirring until blended. Press mixture on bottom and up sides of a 9-inch deep-dish pie plate. Bake 10 minutes. Let cool completely on a wire rack (about 30 minutes).
Whisk together egg yolks, sweetened condensed milk, and lemon juice. Pour into prepared crust.
Bake at 350° for 15 minutes. Let cool completely on a wire rack (about 1 hour). Cover and chill 4 hours.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add powdered sugar, beating until soft peaks form; dollop over chilled pie. Garnish, if desired.