Place cocoa powder and brownie mix in bowl and whisk together.
Add the butter and vanilla and mix well.
Add milk and slowly add the powdered sugar while beating on medium speed. Add the hot water and beat on medium for about a minute.
Adjust the consistency with additional powdered sugar or milk until you get the texture you are after. Use right away or store in an airtight container.
Notes
You can heat treat the brownie mix in the microwave before using it. The instructions are in the post.