Ka'ak b'Nutella (كعك بالنوتيلا) is the sesame street bread filled with chocolate-hazelnut spread, the sweet, modern reading of a loaf that already leans gently sweet. The angle is that this one does not fight the bread, it agrees with it. Where the savory fillings have to balance salt or tang against the ka'ak's faint sweetness and toasted sesame, the chocolate spread runs with both: sweet meeting sweet, hazelnut meeting sesame. That makes it the simplest possible build in the family and pushes all the judgment onto warmth and proportion, because there is no acid or salt doing corrective work.
The construction could not be shorter. The ka'ak is warmed, which matters more here than almost anywhere, because heat softens the crumb and loosens the spread so it goes glossy and molten instead of sitting as a cold stiff stripe. The loaf is split along its wider belly and the chocolate-hazelnut spread is smeared generously inside, then often given a brief return to the heat so it warms through. That is essentially the whole recipe. The judgment calls are temperature and amount. A cold loaf leaves the spread claggy and the sesame flat; a properly warmed one turns it soft and fragrant and lets the toasted sesame read against the sweetness. Too thick a layer and it is cloying with nothing to cut it; a measured amount lets the bread's own mild sweetness and nutty crust stay in the picture. A good ka'ak b'nutella is warm, the spread soft and glossy, the sesame still tasting like itself underneath; a poor one is a cold, oversweet stripe in a stale loaf with the chocolate doing all the talking.
It varies by what is added to lean further into or against the sweetness. Sliced banana is the common partner, bringing a little freshness and body; chopped or whole nuts, a dusting of sugar, or crushed biscuit turn up at some carts. The spread itself ranges from a thin economical smear to a thick indulgent fill. Within the ka'ak family it is the sweet member, sitting opposite the za'atar, cheese, egg, and labneh fillings and sharing the same logic from the other side: instead of balancing the bread's sweetness, it amplifies it. It is the dessert or treat reading of the sesame loaf, the same warm bread put to a sweet end rather than a savory one.