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. Allow crust to cool completely (after cooling, crust can be covered with plastic and held at room temperature for up to 2 days).

Bring cream, 3 Tbsp of sugar and salt to a simmer in a medium saucepan over medium-high heat, stirring occasionally.

Meanwhile, whisk egg yolks, cornstarch and the rest of the sugar together in a medium bowl. When cream mixture begins to simmer, slowly whisk about 1 cup of it into the egg yolk mixture to temper. Slowly whisk the tempered egg yolk mixture into the saucepan with the remaining cream mixture. Reduce temperature to medium and continue whisking until mixture is thickened, about 30 seconds.

Turn off the heat and stir in butter and chocolate until completely melted. Stir in vanilla. Pour warm filling into graham crust. Cover filling directly with plastic and refrigerate while making meringue topping.

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.

Carefully spread meringue over chocolate filling, ensuring to spread all the way to the edges. Dollop with a spatula to form peaks. Bake until peaks are golden brown, 5-7 minutes. Chill until chocolate filling is set, at least 2 hours.