Add chicken bouillon mixture to medium saucepan over medium-high heat; bring to boil. Add chicken; lower heat to medium. Simmer until chicken is cooked through, about 20 minutes. Transfer chicken to plate to cool; shred and set aside.
Meanwhile, heat oven to 180˚F; place baking sheet inside oven. Heat ½” oil in medium skillet over medium-high heat until shimmering, but not smoking. Add one tortilla; cook until crisp and golden brown on both sides, flipping once, about 1 minute. Transfer to paper towel-lined plate to drain, and then transfer to baking sheet in oven. Repeat with remaining tortillas.
To assemble tostadas, spread 3 tbsp. refried beans on each crisped tortilla. Divide reserved chicken, lettuce, tomato and avocado evenly among tortillas. Serve with salsita, pico de gallo, sour cream and cheese, if desired.
To shave 20 minutes off the total time of the Chicken Tostadas recipe, forget cooking the chicken from scratch (step 1). Instead, buy a rotisserie chicken at the grocery store and shred it at home. To do, using two forks, shred the pre-cooked, seasoned chicken, then pile on tortillas.