Happy Rich (F1) broccoli x gailan
Gai lan: Brassica oleracea var. alboglabra
Broccolini is a hybrid between broccoli and gai lan,

Also known as Chinese broccoli, the florets of broccoli x gailon (gai lan) are smaller and more tender than European heading broccolis, and its stems though longer are equally tender and edible, with a sweet, mild flavor. Our hybrid varieties are more vigorous and adaptable than nonhybrids. Top the main shoot to encourage side shoot production over a longer harvest window.
The solution to successful summer broccoli production. Uniform, vigorous, dark-green plants produce jumbo-size florets that resemble mini heads of broccoli, and have an excellent, sweet, broccoli flavor. Produces ample amounts of side shoots when first floret is pinched out and plants are spaced 12-18" apart.

Harvest broccolini when the heads are fully formed but before they begin to flower. Cut long stems; the stem is as tasty as the florets. Leave green leaves on the plant and watch for new heads to form. You may get 3-5 sets of shoots from each plant in any given year.
How to Grow Broccolini - Gardening Channel