By the subjective standards of cheesecake, Basque Cheesecake is burnt, but it is not cooked so long that the top turns to carbon. There are two non-enzymatic browning reactions happening here. The first is the caramelization of the sugar, which creates aromatic compounds such as Diacetyl and Maltol, which give the top the flavor of caramel. The second is Maillard browning, which is a reaction between the proteins in the cream cheese and sugars, which not only creates additional flavor compounds; it also creates the taste of umami. This is why the seemingly burnt layer on top tastes so good.