Combine the cocoa powder, brown sugar, corn syrup, cream, salt and half of the chopped milk chocolate in a small sauce pan and stir. Cook over medium heat, stirring, until chocolate is melted. Cook mixture at a low boil, stirring occasionally for about 5 minutes.
Remove pan from heat, add remaining chocolate, butter and vanilla and stir until smooth.
I transfer the hot fudge to a fun glass jar for easy pouring.
If you have any leftover, it can be refrigerated in an airtight container. All you have to do is reheat it over low heat on the stove or in the microwave.