Make the burnt chilli paste: Heat the oil in a heavy-based frying pan and fry the chillies for a few seconds, without burning them, to release the flavour - add to a blender. Fry the garlic until lightly browned and add to blender. Fry the shallots until lightly browned - add to blender. Blend all the ingredients until smooth.
Return the blender ingredients to the pan with: all the oil used for frying, tamarind puree, fish sauce, sugar/honey, ground shrimp (if using). Fry for a minute or two, until the sugar is dissolved.
Remove all but 2 tablespoons of the burnt chilli paste from the pan. Add the chicken and fry until cooked through.
Add in the fish sauce, stock, sugar and cashew nuts. Stir-fry for 2 minutes. Taste and adjust seasoning, adding more chilli paste if needed.
Spoon onto a serving plate and garnish with Thai basil.