This is based on Internet Shaquille's recipe, but I changed enough things I think my version tastes meaningfully different.
Boil chicken broth, water, and bouillon paste. Remove stems and seeds from the dried chiles, then toast them briefly in a dry pan over high heat (maybe 30 seconds?) before transferring to the boiling liquid. Weigh the chiles down so that they are submerged, turn off the heat, and set a timer for 15 minutes. Use the same pan to char the onion, also over high heat.
Once the timer goes off, add the rest of the ingredients and ensure that the chiles are pliable and hydrated. Remove about half the water. Blend until fairly smooth (ideally in a blender, but I only have an immersion blender, so now my wall has stains on it). Check the consistency: it should be liquid, but thick enough to coat the back of a spoon. If it's too thick, add back some of the reserved water until this consistency is achieved.
If you want a smooth, glossy texture, pass the mixture through a standard (not fine mesh) strainer using a silicone spatula.