Scald the tomatoes and rinse with cold water. Peel, remove the seeds and cut into small cubes. Peel and finely chop the shallot.
Peel the avocado, halve lengthwise and remove the pit. Cube the flesh and mash with a fork. Mix the avocado, shallot, sour cream and diced tomatoes. Season with lime juice, salt and cayenne pepper to taste and serve with tortilla chips.