Whisk egg yolks and lemon zest together in a small bowl until creamy yellow in color, about 2 minutes. Whisk in sweetened condensed milk until smooth, then whisk in the lemon juice. Cover mixture and let sit at room temperature for 30 minutes. While you are waiting, begin preparing crust.

Move oven rack to middle position and heat oven to 325°F. Process graham pieces in a food processor (or crush in a zipper bag with a rolling pin) until they resemble sand. Stir in butter and sugar until mixture is uniform and resembles wet sand.

Press graham mixture into a 9-9.5" pie plate using your hands or the bottom of a dry measure. Bake until crust is fragrant and starting to brown, about 15 minutes. Pour lemon pie filling into crust while it is still warm. Bake until center is firm, but has a slight jiggle when you shake it, about 15 minutes.

With oven rack still at middle position, heat oven to 400°F. Mix sugar and water together in a small saucepan. Bring to boil over medium-high heat and continue cooking until mixture reaches 235°F. Remove from heat and cover to keep warm.

In the bowl of a stand mixer fitted with whisk attachment, add egg whites, cream of tartar, and salt. Whip on medium speed until foamy, about 2-3 minutes. Increase speed to medium high and continue whipping until soft peaks form, 1-3 minutes.

Reduce speed to medium-low and slowly add hot syrup, being careful to avoid the whisk. Add vanilla and turn mixer to medium-high. Continue to mix until mixture is slightly cooled, thick and shiny, 3-6 minutes.

Gently spread meringue over the lemon filling, being careful to spread the meringue all the way to the edges of the pie plate. Dollop the top of the meringue with a spatula to create decorative peaks. Bake until peaks are golden brown, 5-7 minutes. Chill until lemon filling is set, at least 2 hours.