In a small bowl, mix together the créme fraîche, juice from the lime, ancho chile pepper, cayenne pepper and a few pinches of salt. Give it a taste test and adjust flavors according to your liking. Set aside. Note: I personally am obsessed with heat, so I added a few more pinches of cayenne.
Using your grill, char the corn on all sides, rotating every so often to ensure even charring. Alternatively, you could set the corn on the grates of your gas stove with the flame set to medium-high heat, rotating them every minute or so until charred on all sides.
Cut the corn off the cobs and transfer them to a bowl. (Or if you used my method pictured, you can simply take out the small bowl and voila! done!) Add the red onion, serrano peppers and a pinch of salt. Add the dressing and toss lightly, being sure the corn is evenly coated. Give it a taste test and add a bit more salt, cayenne pepper or ancho chile pepper, if you like. Garnish with a small handful of cilantro, queso fresco and a few dash of ancho chile pepper. Refrigerate until ready to serve, up to 1 day.