The phrase "a soft center" is correct and usable in written English.
It can be used to describe something that has a tender or yielding interior, often in the context of food or personality traits.
Example: "The chocolate cake had a soft center that oozed rich molten chocolate when cut into."
Alternatives: "a tender core" or "a gentle heart."
"Maria is steel with a soft center.
I am West Coast frenetic, and Jeff is East Coast hardened, but with a soft center.
Flint corn, which has a soft center and harder outer shell, is what most people know as popcorn.
The bands Elton John took on the road were playing slick pop-rock, with polished surfaces and a soft center.
The plaintive tunes help establish his character: wounded, gruff, thoughtful, an easy touch for a lost cause, a hard-boiled hero with a soft center.
Also appealing was the chocolate cake with a soft center, topped with dulce de leche ice cream and a swirl of caramel.
In front of an enthusiastic audience of patrons and friends at a dress rehearsal later, Mr. Fagan described the three-part premiere: the "Lighthouse" opening and the "Lightning Rod" finale flanking a softer center based on memories that included his favorite part, what he described as "a sophisticated 21st-century ménage à trois".
D. Michael Quinn, author of a two-volume work on the Mormon hierarchy, said that while mainstream members are sensitive to gibes about placing their faith in a gerontocracy, their excitement with Hinckley's strong advent has a softer center.
Crazy as it sounds, Burning Man has a center, a soft core.
Tengda offers Western desserts that are top-notch, including a tangy Key lime tart, a brownie-ice-cream-honeyed- walnut-amaretto cookie fantasy, lightly deep-fried crispy coconut cream, and the "chocolate delight" (a round individual cake with a seductive soft center).
Stonehenge's signature soufflé, in this case Grand Marnier with a crème anglaise sauce, was up to its usual high standard, as was a decadently rich chocolate cake with a warm, soft center.
