Instructions

for the filling

Microwave milk chocolate and bittersweet chocolate in large bowl at 50 percent power, stirring occasionally, until melted, 2 to 4 minutes. Whisk egg yolks, sugar, and salt together in medium bowl until combined, about 1 minute. Bring half-and-half to simmer in small saucepan over medium heat. Whisking constantly, slowly add hot half-and-half to egg yolk mixture in 2 additions until incorporated. Return half-and-half mixture to now-empty saucepan and cook over low heat, whisking constantly, until thickened slightly, 30 seconds to 1 minute. Stir half-and-half mixture into melted chocolate until combined. Let cool slightly, about 8 minutes.
Using stand mixer fitted with whisk, whip cream on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, 1 to 3 minutes. Gently whisk one-third of whipped cream into cooled chocolate mixture. Fold in remaining whipped cream until no white streaks remain. Cover and refrigerate for at least 3 hours, or until ready to assemble pie. (Filling can be made up to 24 hours in advance.)

for the meringue crust

Adjust oven rack to lower-middle position and heat oven to 275 degrees. Grease 9-inch pie plate and dust well with extra cornstarch, using pastry brush to distribute evenly. Combine sugar and 1 tablespoon cornstarch in bowl. Using stand mixer fitted with whisk, whip egg whites and cream of tartar on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip whites to soft, billowy mounds, 1 to 3 minutes. Gradually add sugar mixture and whip until glossy, stiff peaks form, 3 to 5 minutes. Add vanilla to meringue and whip until incorporated.
Spread meringue into prepared pie plate, following contours of plate to cover bottom, sides, and edges. Bake for 1½ hours. Rotate pie plate, reduce oven temperature to 200 degrees, and bake until completely dried out, about 1 hour longer. (Shell will rise above rim of pie plate; some cracking is OK.) Let cool completely, about 30 minutes.

for the topping

Spoon cooled chocolate filling into cavity of pie shell, distributing evenly. Using stand mixer fitted with whisk, whip cream and sugar on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until stiff peaks form, 1 to 3 minutes. Spread whipped cream evenly over chocolate. Refrigerate until filling is set, about 1 hour. Dust with cocoa. Slice with sharp knife and serve.