Position your oven rack about 6 inches from the broiler element and turn the broiler to high. Line a baking sheet with foil for easy cleanup.

Combine the drained black beans, corn, cumin, garlic powder, and salt in a small saucepan over medium heat. Stir and cook for 3 to 4 minutes until the beans are warmed through and fragrant. Taste and add more salt if needed. You want the mixture warm so it does not cool the shells before the cheese melts.

Arrange the mini tostada shells in a single layer on the prepared baking sheet. Spoon about a tablespoon of the warm bean and corn mixture onto each shell. Do not pile it too high or the topping will slide off under the broiler.

Sprinkle a generous pinch of shredded cheese over each tostada, covering the bean mixture. The cheese acts as a glue that holds the toppings in place once it melts.

Slide the baking sheet under the broiler and watch closely. After about 1 1/2 to 2 minutes, the cheese should be fully melted and bubbling, and the edges of the shells should look golden and a little darker. Pull them the moment the edges start to deepen in color because they go from toasty to burnt very quickly.

Let them cool on the pan for one minute before serving. Set out sour cream and salsa on the side and let kids add their own. Serve immediately while the shells are still crisp.