Apparently, please correct me if I'm wrong, Cadbury's put a solid filling in their creme eggs. Then, after the 2 parts are joined together they inject an enzyme which turns the centre gooey.
As stated, Creme Eggs are made by filling moulded shells. You may be thinking of 'soft-centre' sweets.
The sugar-based centre filling, which initially has a solid consistency, is mixed with an enzyme before dipping in chocolate to form the sweet. Over time, the enzyme converts some of the sucrose into invert sugar, to form a soft or even liquid centre, according to how much is added.
A comment, not an answer.
That's reallt interesting because sometimes you do get a more solid filling, I always thought they had been exposed to the air but probably not.