Broccolini is a hybrid between plain broccoli and a species of Chinese cabbage. It is not grown for the large flower heads as in broccoli, but more for the very delicate stems with a taste that is a cross between broccoli and asparagus. It is said that it is popping up in some stores over time, but is still a bit of a luxury vegetable as it is somewhat more expensive to produce than regular broccoli.
Broccolini is seen in April, where you then grab the main shoot as early as possible - and then the plant begins to form side shoots that form the actual crop. These are cut when they are 15-20 cm long, which you can continue with for several months. The shoots of course become smaller and smaller with time, and in total you can harvest a 500-600 gr per plant.
This year I have grown the varieties Apollo (the regular variety for several years) and Sibsey (new). Sibsey is a somewhat stronger plant, which has also coped with the frost best - and which also apparently continues to give despite temperatures several nights of minus 6-7 degrees. The new shoots are probably not nearly as big as they have been, but a small handful can still be cut a week to throw into the wok.