Step 1Heat milk and chocolate together in saucepan over low heat until chocolate is melted, stirring occasionally.
Step 2Combine sugar, salt, cornstarch, flour and cocoa in saucepan. Gradually stir in hot milk mixture. Cook over medium heat, stirring constantly until mixture thickens and reaches a boil. Boil 1 minute. Remove from heat. Stir 1 cup hot mixture into beaten egg yolks. Return all yolk mixture to saucepan. Bring just to simmer, stirring constantly, until custard thickens slightly. Remove from heat, and stir in butter and vanilla. Pour custard into bowl. Cover with plastic wrap, pressing plastic directly on top of custard. Cool slightly so it is no longer hot but still warm. Spoon custard into cooled Master pie shell, spreading evenly.
Step 1Combine water and sugar in small saucepan. Cook over medium heat until syrup reaches 238 degrees on a candy thermometer. Remove from heat.
Step 2Beat egg whites to form soft peaks, 30 seconds to 1 minute. With mixer running at medium speed, add syrup in slow stream. Continue beating until meringue is cool and stiff peaks form, about 5 minutes. Spoon meringue into pastry bag with large tip for piping. Pipe in decorative pattern over top of tart, spreading to edges.
Step 3Bake at 425 degrees until meringue is lightly browned, about 10 minutes. Sprinkle with hazelnuts last few minutes of baking. Cool pie 1 to 2 hours before serving.