KOSHER SALT & GROUND BLACK PEPPER
SOUR CREAM OR FULL-FAT YOGURT
Add all listed ingredients (minus the water) to a food processor, seasoning with a heaping 1/2 teaspoon kosher salt & ground black pepper as desired. Pulse to break up the avocado & cilantro, then blend until smooth & creamy. If necessary, add a couple tablespoons water to smooth out texture.
Place the sweet corn directly over a gas burner. Light to a medium-high flame & cook about 2 minutes per side or until as charred as desired. Transfer to a cutting board & carefully slice kernels away from the cob. Set aside. (If you do not have a gas stove, you can also char the corn on an outdoor grill, in a grill pan, or under the broiler.)
Meanwhile, as the corn chars, get the black bean tostada topping cooking. Add the olive oil to a medium skillet over medium heat. Once hot & shimmering, add the onion. Season with 1 teaspoon kosher salt. Cook, stirring occasionally, until translucent & fragrant, 3-4 minutes. Stir in the garlic. Cook just until fragrant, about 30 seconds. Add in the chili powder, ground cumin, & smoked paprika. Stir, coating the onion. Cook 1-2 minutes longer, until very fragrant.
Add the black beans to the skillet, stirring to combine with the aromatics. Cook, stirring occasionally, until warm, 1-2 minutes. Add the vegetable broth/water & increase heat to bring the black bean mixture to a simmer. Use a spatula or wooden spoon to mash the black beans slightly, binding everything together. If needed, cook 1-2 minutes longer to thicken slightly. Squeeze the lime juice over the black bean mixture. Remove from the heat & set aside for tostada assembly.
If desired, warm tostada shells according to package directions. Spread the prepared black bean topping over each tostada shell. Top with charred sweet corn kernels & a drizzling of avocado crema over top. If desired, add other toppings like chopped cilantro, cotija cheese, shredded lettuce, etc.