Place kale in salted boiling water for 8 minutes, until tender and bright green. Reserving the boiling water, remove with a slotted spoon, drain well and squeeze out excess water, then chop fine.
Boil the sweet potato in the water used for the kale for 15 minutes, covered, or until tender. Drain and pass through a potato ricer or food mill, into a large, heat-proof pot. (Note: If you have neither a ricer nor mill, push potato through a sieve or otherwise be sure to mash thoroughly until smooth.)
Add the chopped kale to the sweet potato. Season with salt and pepper to taste, keeping in mind that the inclusion of cayenne and pancetta later in the recipe will boost the levels of both salt and spice; be judicious.
In a medium skillet over moderate heat, cook the pancetta until crisp but not browned. Remove using a slotted spoon and let drain on a paper towel.
In the same pan you used to cook the pancetta, melt the butter. Add onion and cayenne and sauté over medium heat, until onion is translucent and has lost its crunch. Set aside.
In a small saucepan, heat half-and-half, then beat it into the sweet potato-kale mixture (I just used a fork, which worked best). Add the pancetta. Add the onion and the fat from the pan. Combine all ingredients well, and serve warm.
This dish may be prepared in advance. In fact, it tastes even better the next day. Also, try pressing the colcannon into a pie plate and chilling overnight: The next day, cut into wedges and fry up in additional butter or oil to accompany a breakfast of scrambled eggs.