Heat a dry cast iron pan, comal or grill to high. No oil and no butter yet; the corn chars dry.
Char the husked ears, turning every 2 to 3 minutes, until blackened in patches on all sides, 10 to 12 minutes total. Kernels should look blistered in spots, not uniformly brown.
Stir the mayonnaise, crema and grated garlic together while the corn cooks.
Brush or spoon the mayo-crema mix over the hot ears the moment they leave the heat, so it melts into the kernels.
Shower each ear with crumbled cotija, pressing gently so it sticks, then dust with chili powder.
Finish with chopped cilantro and serve immediately with lime wedges. The lime goes on at the table, squeezed over every few bites.