What's in a name? Broccolini is a Trade Marked name for a hybrid cross of Broccoli and Kailaan, Chinese Broccoli. They are so closely related it can't have been difficult to achieve this cross but the breeding appears to be relatively recent. It's best to think of Broccoletti as a member of the "Sprouting Broccoli" group.
We could have chosen any number of names: Tender Stem (my favourite), Kaibroc, Baby Bunching or Broccolette but we can't use the name that has become the standard in the supermarkets... Great Marketing.
Broccoletti has a sweet Broccoli flavour and "hints of Asparagus". I'm personally suspicious of the Asparagus flavour claim but the thin stems make a perfect Asparagus substitute, they are very tasty and much more digestible than the thick, hard stems of regular Broccoli. So tasty that I'm tempted to cut the florets off and just eat the stems.
Stephanie Alexander calls "Sprouting Broccoli" a "very special and luxurious vegetable". Pick when young and tender, "steam it for a few minutes or cook it briefly... and serve hot with Hollandaise Sauce". That's not luxurious that's extravagant!