1. Preheat the oven to 375°F. Fit one 9-inch pie shell into a deep 9-inch pie pan.
2. Fill a bowl with cold water and add the lemon juice. Peel, core and slice the apples. Drop them into the cold water as you slice them.
3. When all the apples are sliced, drain them, discard the water, and return the apples to the bowl. Toss with the vanilla, both sugars, cinnamon, flour, and salt. Mound into the pie shell.
4. Cut the butter into small pieces and scatter it over the apples.
5. Arrange a second pie shell over the apples. Crimp edges with a fork. Cut out a small hole from the center of the pie to vent the steam.
6. Place the last pie shell onto a lightly floured work surface. Using a small, leaf-shaped cookie cutter, cut out leaves. Reroll the scraps of dough and cut out more leaves. Brush one side of each leaf with water. Arrange the leaves on top of the pie. Don’t cover the center hole in the pie with leaves. (see video below)
7. In a small bowl, beat together the egg yolk and the water. With a pastry brush, brush the leaves with the egg wash.
8. Bake the pie for 25 minutes. Reduce the oven heat to 350°F and continue to bake the pie for another 35 to 40 minutes, until the pastry leaves are lightly golden and the fruit juices are bubbling. Cool the pie for at least an hour before serving. Serve with softened vanilla ice cream or whipped cream, if desired.