Make the ketchup. Heat the oil in a small saucepan, add the shallot and cook for 5 minutes until softened. Add the garlic and cook for a further 30 seconds. Stir in the tomatoes and continue to cook for 3 minutes, crushing them with the back of a spoon. Add the vinegar, sugar and chipotle paste and simmer for 20 minutes until the liquid has evaporated. Season and leave to cool.
Rub the oil over the chicken and season well. Heat a griddle pan until smoking hot, add the chicken and cook for 10–15 minutes, turning once, until charred and cooked through. Remove from the pan and keep warm.
Place the buns, cut-side down, in the griddle pan and cook for 1 minute until toasted. Remove from the pan. Add the chorizo slices to the pan and cook for 30 seconds until starting to crisp.
Arrange the chicken breasts on the buns. Top with the chorizo, avocado slices, a spoonful of soured cream and a dollop of the chipotle ketchup.
Serve the burgers with potato chips dusted with a little chilli powder.