Wash and dry produce. 

Dice potatoes into ½-inch pieces. Drain corn and pat dry with paper towels. Dice onion into ½-inch pieces. Pick leaves from cilantro; mince stems. Quarter lime. Reserve ¼ tsp chili powder (½ tsp for 4 servings) in a small bowl (you'll use it in Step 4).

Place potatoes in a medium bowl with 1 TBSP water (2 TBSP for 4 servings). Cover tightly with plastic wrap and microwave until slightly tender, 5-7 minutes. (They'll finish cooking in the soup.)

Heat a drizzle of oil in a large pot over medium-high heat. 

Add corn, onion, and cilantro stems. Cook, stirring occasionally, until onion is softened, 3-4 minutes. 

Add garlic powder, remaining chili powder, and ¼ tsp sugar (½ tsp for 4 servings). Cook, stirring, until fragrant, 1 minute.  

Add potatoes (careful when uncovering the bowl—steam may release!), stock concentrates, 1½ cups water, and ½ tsp salt (3 cups water and 1 tsp salt for 4 servings) to pot. 

Bring soup to a boil, then reduce heat to low and cover. Simmer until soup has thickened, 5-7 minutes. 

Turn off heat; stir in cream sauce base, half the cotija, 1 TBSP butter (2 TBSP for 4), and a squeeze of lime juice. 

Divide soup between bowls; top with reserved chili powder, remaining cotija, cilantro leaves, pepitas, and a squeeze of lime juice. Serve with remaining lime wedges on the side.