Begin by melting unsalted butter in a skillet over medium heat. Add the finely chopped onion and sauté until translucent, about three minutes. Stir in minced garlic along with cumin and smoked paprika; cook for an additional minute until fragrant.
Add the refried black beans into the skillet and stir well to combine them with the spices and aromatics. If desired, mix in hot sauce and salt according to your taste preferences while heating through.
On a sturdy plate or platter, start by placing two tostada shells side by side as your base layers. Spread half of the creamy black beans onto each shell evenly before adding shredded chicken on top.
Sprinkle shredded cheese generously over the layered chicken on both tostadas until fully covered. If you’re feeling indulgent, feel free to add more cheese than specified.
Top each stack with fresh salsa followed by dollops of sour cream and mashed avocado as desired. Finish off with fresh cilantro leaves scattered on top before serving immediately while everything is warm and inviting.