Clarified butter is butter oil from which all milk solids have been removed. It keeps longer than normal butter, and has a higher burning point, making it better for frying. It is becoming my new favorite thing, as the milk solids are what most people with problems with dairy are sensitive to, including Baby.
Most methods of making clarified butter involve cooking it on the stovetop. This requires attention, which I often have in relatively short supply. However, it's dead easy to do in a slow cooker.
Take some butter. Put it in a jar (you may be able to add some more once the first bit melts, if you can't fit multiple solid sticks in the jar). Put the jar in a crock pot, and fill the crock pot with water until at least half full (obviously, don't cover the mouth of the jar). Turn on low overnight or longer. The milk solids will rise to the top and be all foamy, or (depending on how long you cook it) settle to the bottom.
(I've seen instructions for doing this by putting butter straight into one of those little 1-2 qt crock pots, but we don't have one, so the water bath works for us.)
In the morning, spoon any foam off the top, then pour the butter oil through a fine strainer or cheesecloth into another container. If you want to be extra safe, do this very gently, and don't pour the last bit with the settled milk solids.
Baby tolerates the results without any obvious issue. I wouldn't suggest someone with a more serious allergy try it. Particularly not a life-threatening one.