Heat a large skillet over medium heat and add the oil. When shimmering, add the corn and jalapeno and cook for 1-2 minutes, until the kernels start to lightly pop.
Stir, letting cook another 2-3 minutes until slightly charred, then add in the salt and garlic cloves and cook another 30 seconds, taking care not to burn.
Remove from heat and let slightly cool, then stir in the sour cream, lime juice, mayo, chili powder, salt, cotija cheese, cilantro and scallions. Taste, adjust for preference-- adding in more lime juice/salt/cheese as desired.
Spread a few tablespoons of the warmed refried beans onto each tostada shell then top with the corn mixture. Serve!