Combine the brown sugar, smoked paprika, oregano and cumin in a small bowl.
Place the turkey thigh in a slow cooker and rub the spice mix all over it.
Add the ancho chilli paste, chipotle paste, garlic, tomatoes, onions, cinnamon stick and tomato purée. If using, add the dried guajillo chillies.
Pour in enough chicken stock to just cover the ingredients.
Cook on high for 5–6 hours or low for 8–10 hours, until the turkey is very tender.
Remove the turkey from the slow cooker and shred the meat, discarding the bones.
Strain the cooking liquid through a sieve, pressing on the solids to extract as much liquid as possible.
Heat a splash of oil in a large skillet over medium-high heat.
Dip the corn tacos in the strained cooking liquid, then fry for 1–2 minutes per side until crisp.
Top the tacos with the shredded turkey, some of the cooking liquid, the cheeses, onions and coriander.
