Mix cocoa, cornstarch, beaten egg yolks, sugar and salt; then add milk slowly, while stirring in a pot over medium-high heat and cook until thick, beating until smooth. Cream pies take a while to thicken up so stir and stir (about 10-15 minutes). Pour into a pre-baked pie shell. Refrigerate to chill. Top with whipped cream or make a meringue using left over egg whites.