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For the potatoes: Place the potatoes in a pot with water to cover. Salt the water and bring to a boil. Cook the potatoes until almost cooked through; a paring knife should meet with a little resistance when inserted into center. Drain the potatoes, cool slightly and cut into medium dice, then reserve for later use.
Heat the oil in a large nonstick saute pan over high heat until it begins to shimmer. Add the chorizo and cook until golden brown and cooked through, about 8 minutes. Transfer the chorizo to a plate. Remove all but 2 tablespoons of the fat from the pan. Add the onions and cook until soft, about 4 minutes. Add the butter to the pan, melt, and then add the potatoes. Toss well, season with salt and pepper, add the ancho and cook, pressing the mixture into the pan occasionally, until lightly golden brown and crisp. Stir in the chorizo. Remove from the heat, stir in the chopped cilantro and transfer to a large shallow bowl or platter.