Preheat oven to 325 degrees F. Lightly coat a 9-inch pie plate with cooking spray; set aside. In a medium bowl stir together crushed cookies and the melted butter; press onto the bottom and up the sides of the prepared pie plate. Chill about 1 hour or until firm.
Separate eggs. Place egg whites in a large mixing bowl and let stand at room temperature for 30 minutes. Place egg yolks in a small bowl and beat lightly; set aside.
In a medium saucepan combine 1 cup of the sugar and the cornstarch. Gradually stir in half-and-half and chocolate. Cook and stir over medium-high heat until thickened and bubbly; reduce heat. Cook and stir for 2 minutes more. Remove from heat. Gradually stir about 1 cup of the hot filling into the egg yolks. Add yolk mixture to saucepan. Bring to a gentle boil, stirring constantly; reduce heat. Cook and stir for 2 minutes more. Remove from heat. Stir in the 1 tablespoon butter and 1 1/2 teaspoons of the vanilla. Keep filling warm.
For meringue, add cream of tartar and the remaining 1 teaspoon vanilla to egg whites. Beat with an electric mixer on medium speed until soft peaks form (tips curl). Gradually add the remaining 1/2 cup sugar, 1 tablespoon at a time, beating on high speed until mixture forms stiff, glossy peaks (tips stand straight) and sugar dissolves.
Pour warm filling into prepared piecrust. Spread meringue over warm filling, sealing to edge of crust. Lightly sprinkle cocoa powder over the meringue. Bake for 30 minutes. Cool on a wire rack for 60 minutes. Chill for 3 to 6 hours before serving; cover for longer storage.