The outside of the cake is just a little crisp and the center just melts in your mouth.
melt in (one's)/the mouth
Of food, to be soft or tender, and, typically, delicious. The phrase is often quite literal. This decadent chocolate cake positively melts in the mouth.I've never had such a perfect steak in my life! I swear, it melted in my mouth.
ˌmelt in your ˈmouth
(of food) be soft and very good to eat: They serve steaks that just melt in your mouth.