In a heavy saucepan, whisk together ½ cup sugar, cocoa, ¼ cup cornstarch and 1¾ cups half-and-half. Place over medium-high heat and stir until sugar is just dissolved, about 2 min. Whisk in 4 egg yolks; reduce heat to medium. Cook, stirring constantly, until custard thickens and just boils, about 5 min. Remove from heat and stir in chocolate and 1 tsp vanilla. Place pan in a bowl of ice and water and stir occasionally until custard cools.
Remove from ice bath, dry pan bottom and transfer custard to prepared crust. Cover with plastic wrap, gently pressing wrap onto surface of custard; refrigerate.
In another heavy saucepan, stir together ¼ cup sugar, ¼ cup cornstarch and 1¾ cups half-and-half. Place over medium-high heat and stir 2 min. Reduce heat to medium and whisk in 4 egg yolks. Cook, stirring constantly, until custard thickens and just boils, about 5 min. Remove from heat and stir in 1 tsp vanilla and butter. Cool custard as above over an ice bath, then spoon on top of chocolate filling and smooth. Cover with plastic wrap and refrigerate until set, at least 3 hrs.
Whip cream and confectioners' sugar until firm peaks form and spread onto chilled pie. Garnish with chocolate curls or shavings if desired.
That extra punch of chocolate holds up well against the vanilla to bring a just-right flavor!