Toast the chiles in a hot skillet over medium-high heat 2-3 minutes per side.
Remove to a large bowl and cover in hot water. Let sit for 30 minutes, turning once.
Remove chiles (do not discard the water). Cut off the stems and remove the seeds.
Chop lightly and transfer to a food processor.
Add the cumin, pepper and a few pinches of salt, along with 1/4 cup of the soaking liquid. Process until smooth. If needed add a bit more of the water.
Heat the oil in a large Dutch oven over medium heat.
Add the onion and cook for 3 minutes.
Add the garlic and cook for another minute.
Add the broth and water. Bring to a simmer.
Whisk in the masa harina and add the processed chile paste.
Add the brisket and stir. Cover.
Let simmer for 2 hours or until the beef is tender, adding more broth if necessary.
Stir in the brown sugar and vinegar and add more salt to taste. Simmer another 10 minutes.
Serve with the desired garnishes.