For the Pudding Pie Filling: Whisk sugar, cocoa, cornstarch and salt together in a medium sized saucepan (if it has rounded bottom edges, all the better to whisk every bit of the pudding). Slowly whisk in about 1/2 cup (120 ml) of the half-and-half (you can do this by eye) until combined; the cocoa will resist a bit. Just keep whisking, then whisk in remaining half-and-half. Cook over low-medium heat, whisking often, until pudding begins to thicken and you can see whisk marks, about 3 to 5 minutes. Add chocolate, whisk a few times until chocolate is melted and pudding is thick, then remove from heat. Whisk in vanilla and pour into prepared pie crust, leveling the top. Cool at room temperature for about 1 hour, then refrigerate at least 4 hours.