Dad's Favorite Chocolate Pie — Three Many Cooks

Ingredients

• 1 9-inch pre-baked pie shell
• 10 tablespoons plus 1/2 cup granulated sugar, divided
• 1/4 cup, plus 1 tablespoon cornstarch, divided
• 2 tablespoons unsweetened cocoa powder
• Pinch salt
• 5 large eggs, separated
• 1 can (12 ounces) evaporated milk
• 1 cup whole or 2% milk
• 4 ounces bitter- or semi-sweet chocolate, coarsely chopped or broken
• 2 tablespoons butter
• 1 1/2 teaspoons vanilla extract
• 1/4 teaspoon cream of tartar