In a high-speed blender or bullet, add orange juice and achiote paste. Blitz until thoroughly combined and slightly thickened.
Devein prawns and slice up the middle lengthwise.
In a bowl, combine prawns and marinade. Cover and refrigerate for 2 hours.
Bring a pot of salted water to the boil. Fill a bowl with iced water.
Drop coriander (leaving stalks on) into boiling salted water for 15 seconds then remove and place in iced water. Strain off water and dry coriander on kitchen paper towels.
Add coriander, tomatillo purée, onion, epazote (if using) and chilli to blender. Process until mixture thickens (the tomatillo seeds break down and act as a thickener). Season with salt to taste and set aside.
In a bowl, add avocado and mash with a fork. Add chilli, onion, garlic, coriander and tomato and mix well. Stir in lime juice and Tabasco. Season with salt to taste and set aside.
Drain prawns, reserving marinade. In a frying pan over medium heat, add oil and heat until shimmering. Add prawns and fry until just cooked.
Add marinade to pan, remove prawns and reduce liquid until thick enough to coat prawns. Return prawns to pan, stir well then remove from heat and set aside.
Fill taco shells with guacamole followed by spring onion, coriander leaves then prawns. Top with tomatillo sauce and serve (or serve sauce on the side).