INSTRUCTIONS
[FOR THE FILLING]
1. Microwave milk chocolate and bittersweet chocolate in large bowl at 50 percent power, stirring occasionally, until melted, 2 to 4 minutes.
2. Whisk egg yolks, sugar, and salt together in medium bowl until combined, about 1 minute.
3. 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.
4. Return half-and-half mixture to now-empty saucepan and cook over low heat, whisking constantly, until thickened slightly, 30 seconds to 1 minute.
5. Stir half-and-half mixture into melted chocolate until combined.
6. Let cool slightly, about 8 minutes.
7. 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.
8. Gently whisk one-third of whipped cream into cooled chocolate mixture.
9. Fold in remaining whipped cream until no white streaks remain.
10. 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]
11. Adjust oven rack to lower-middle position and heat oven to 275 degrees.
12. 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.
13. 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.
14. Add vanilla to meringue and whip until incorporated.
15. 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.)
16. Let cool completely, about 30 minutes.
[FOR THE TOPPING]
17. Spoon cooled chocolate filling into cavity of pie shell, distributing evenly.
18. 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.
19. Spread whipped cream evenly over chocolate.
20. Refrigerate until filling is set, about 1 hour.
21. Dust with cocoa.
22. Slice with sharp knife and serve.
