Preheat oven to 450.
Coat both sides of the tortillas with cooking spray (I use coconut oil spray). Stack and cut into 8 wedges; push into cups of standard muffin pans (see photo above). Bake 6-8 minutes, or until golden.
Mash together avocado, tomato, lime juice, garlic, cilantro, and salt to taste; chill in the refrigerator.
Simmer ketchup, brown sugar, chipotles, soy sauce, vinegar, chili powder, and chicken in a medium-sized pot over medium-low heat for 8 minutes or until sauce is syrupy.
Top each tostada with 1 tbsp guacamole, then 2 tbsp chicken mixture. Garnish each tostada with sour cream, cilantro, and lime zest.
Source: Slightly adapted from Cuisine at Home, December 2007