In a food processor, process the beans until smooth.
Put beans in a large saucepan, mix with 1 tsp chili powder, 1/2 tsp cumin, salsa and chillies. Heat until bubbling, then set aside.
In a lightly greased skillet, place chicken, then sprinkle with remaining cumin and chili powder and heat over low heat, stirring until warm.
Spread the bean mixture over a tostado, then top with chicken, cheese and tomatoes in that order. Repeat until all tostados are used. Place on a baking sheet and put into preheated, 350 degree oven for about two minutes or until cheese melts.
Remove from oven, sprinkle each tostado with lettuce and top with sour cream.