Preheat your oven to 375°F. Place the tostada shells on a baking sheet and warm them in the oven for about 5 minutes until crispy.
In a medium bowl, combine the shredded chicken, chili powder, cumin, salt, and pepper. Mix well until the chicken is evenly coated with the spices.
Spread a layer of refried beans on each tostada shell.
Top the beans with the seasoned chicken mixture.
Add a generous amount of shredded lettuce and diced tomatoes on top of the chicken.
Sprinkle shredded cheddar cheese over the toppings.
Return the tostadas to the oven for an additional 5-7 minutes, or until the cheese is melted and bubbly.
Remove from the oven and drizzle with sour cream and salsa. Garnish with fresh cilantro if desired.