Chocolate Pie Filling

from Louise Lalande (Mom) by way of an old cookbook

1. In a large microwave safe mixing bowl, combine 3 1/2 cups of the Milk, Sugar, Cocoa Powder, Vanilla Extract, and Salt. Microwave intermittently, stirring between heats, until hot.
2. In a small bowl combine the remaining 1/2 cup Milk and the Corn Starch. Mix well to create a slurry. Add to the heated filling mix, stirring well while adding.
3. Continue microwaving the filling mixture until thickened and all the Corn Starch is cooked and gelatinized.
4. Add filling to a blind baked pie shell or graham cracker crumb pie shell. Set aside to cool to room temperature and set.
5. Serve with whipped cream.