Using a sharp knife, carefully cut corn kernels off cob. Add about 1/2" water to a medium saucepan and salt well. Bring to a simmer. Add corn, cover, and cook until corn is tender, 3 to 4 minutes. Drain and pat dry.
In a serving bowl, toss corn with mayonnaise, cotija, lime juice, cilantro, and chili powder. Season generously with salt.
Top with more cotija, cilantro, and a sprinkle of chili powder before serving.