For the crust: In a food processor, pulse the graham crackers and sugar until crumbs form. Pour in the butter and continue to pulse until the crumbs are coated. Press the mixture firmly into the bottom of a 9-inch pie plate. Set aside.
For the filling: In a small saucepan, heat the milk until hot but not simmering. To a medium saucepan, add the sugar, flour and cocoa and whisk together well. Lightly beat the egg yolks (saving the whites for the meringue) and pour into the flour mixture. Slowly add the hot milk, stirring. This will create a paste in the beginning, ensuring that the chocolate and flour blend smoothly. Place over medium heat and cook, stirring constantly, until very thick, about 5 minutes. The mixture will start to bubble sporadically; continue to stir and cook for another 2 minutes. Remove the pan from the heat and stir in the butter, vanilla and salt. Stir until the butter is completely incorporated.
Pour the filling into the crust and chill in the refrigerator, about 45 minutes.
For the meringue: Preheat the oven to 325 degrees F. With an electric mixer, beat the egg whites with the salt until stiff. With the mixer running, gradually add the sugar and beat until the whites are smooth and glossy. Spread the meringue on the pie, reaching the edges of the crust to seal in the filling. Bake until brown, 15 to 20 minutes. Cool completely before serving.
Adapted from "Georgia Cooking in an Oklahoma Kitchen" by Trisha Yearwood (c) Clarkson Potter 2008. Provided courtesy of Trisha Yearwood. All rights reserved.