Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper.
Step 2: In a large bowl, toss the diced sweet potatoes with 2 tablespoons of olive oil, 1 teaspoon of salt, and 1/2 teaspoon of black pepper. Spread them in a single layer on the prepared baking sheet.
Step 3: Roast for 25-30 minutes, or until tender and lightly caramelized, flipping halfway through. Set aside.
Step 4: If using fresh corn on the cob, grill or char it in a cast-iron skillet until kernels are lightly browned and slightly smoky. If using frozen corn, heat the remaining 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the corn and cook for 7-10 minutes, stirring occasionally, until it starts to char in spots.
Step 5: While the corn is still warm, transfer it to a medium bowl. Add mayonnaise, 2 tablespoons chopped fresh cilantro, lime juice, chili powder, and cayenne pepper (if using). Mix well until the corn is evenly coated.
Step 6: Stir in the crumbled Cotija cheese. Taste and adjust seasoning, adding more lime juice, chili powder, or salt if needed.
Step 7: Divide the roasted sweet potatoes among four serving bowls.
Step 8: Spoon a generous amount of the Mexican street corn mixture over the sweet potatoes in each bowl.
Step 9: Garnish with additional fresh cilantro, a sprinkle of Cotija cheese, a lime wedge, and a dash of hot sauce if desired. Serve immediately.