Remove any excess fat from your chicken thighs. Feel free to cut them in half and place them in a bowl (4 pieces total).
Take a fork and prick the thighs all over. This will help the marinade seep into the chicken.
Add your tandoori masala (store-bought or homemade), garam masala, ginger-garlic paste, buttermilk, and salt. (Note - store-bought tandoori masala already contains quite a bit of salt so use a smaller amount like I did here. If you are using your own, then feel free to add more salt.) Combine and cover the bowl with plastic wrap. Set aside in the fridge for 6-8 hours.
Let's make the dredge. In a separate bowl, add your flour and spices (garlic powder, onion powder, chili powder, paprika, and salt). Whisk together. Take a few teaspoonfuls of the marinade from the chicken and also whisk into the flour.
Take each piece and coat in the flour mixture on both sides. Feel free to coat twice for extra crispiness.
Deep fry the chicken in oil - internal temp of chicken should be 170 F.
For the achaari mayo - combine the mayo, achaar, and honey.
Toast your buns, add your mayo, chicken, and top with some shredded cabbage and onion.