"I'd hardly know where to begin with anything else." He can say that much here and sound like nothing more than someone whose parents never taught him to cook for himself - which is true, even if the reasons are more complex (and a little Statute-breaking to bring up in a non-magical grocery) than that.

His mother certainly doesn't know how to cook, either. Regulus is admittedly not sure about his father. Kreacher is also an excellent cook if he happens to like you, and probably wouldn't have allowed Regulus into the kitchen to make the attempt in the first place.

"A cake does sound like an excellent idea, though."