To answer your question, the taste of a cake made with bread or a bread-based recipe would likely depend on the specific ingredients used and the preparation method. If the recipe includes typical cake ingredients like sugar, eggs, and flavorings such as vanilla, it might have a cake-like taste, but the bread's natural flavor could still be present to some extent.
If "bread cake" involves spreading Nutella on bread, then the taste would primarily be influenced by the Nutella spread itself, which is a hazelnut cocoa spread. The bread would likely provide a neutral base for the Nutella's flavor to shine.
To get a definitive answer on how "bread cake" tastes, it would be best to look for specific recipes or try it yourself if you come across it. Food preferences can be subjective, so taste experiences might vary from person to person.