Melt butter in a large saucepan over medium heat. Add onions, a healthy pinch of salt, and seasonings (smoked paprika, cumin, and cayenne). Cook, stirring occasionally, for 3-4 minutes until onions are softened.

Stir in garlic and cook for one minute. Add refried black beans, and stir to mix into the butter, onions and garlic.

Stir the beans occasionally until they are warmed through and their texture becomes soft and creamy. Stir in the sour cream and hot sauce, then turn off the heat from underneath the saucepan.

Toss shredded chicken with your favorite salsa until the chicken is thoroughly coated. Set aside.

Spread about ¼ cup of creamy refried black beans over the surface of one tostada shell. Sprinkle about 2 tablespoons of chopped green onions over the black beans, then place a second tostada shell on top.

Spread another ¼ cup of creamy refried black beans over the second tostada shell, then pile ½ cup of shredded chicken on top. Sprinkle ¼ cup of cheese over the chicken.

Repeat with the other two tostada shells to create the second stack.

Turn on the oven to a low broil. Once it is hot, pop the tostada stacks onto a rimmed baking sheet and broil for 3-4 minutes until the cheese is melted.

For an extra browned / crispy cheese layer, set the broiler to 'high' for about a minute until the cheese is browned.