The Whizz Pop Chocolate Shop by Kate Saunders
The Spoffard family has just inherited their great-great uncle's chocolate shop. Twins Oz and Lily are suddenly thrust into the adventure of a lifetime. It begins by Lily meeting Demerara, the invisible, immortal talking cat, and Oz meeting Spike, the immortal talking rat. Demerara was the cat of their great-great uncle Pierre who was killed in a freak tram accident in the 1930s. The twins and the animals enlist the help of Caydon, a boy across the street, to find the missing chocolate recipe that makes people (and animals) immortal. The kids become agents to London's top secret agency the SMU, the Secret Ministry of the Unexplained, to fight the evil gang that ts after the secret recipe.
If you are a fan of Harry Potter and Men in Black, this is the book for you. It reminds me of Men in Black because it has different departments that deal with weird creatures and the agencies can also erase the minds of people when crazy things happen (wait until you read about what happens to Demerara when she eats a special chocolate bean). It is also full of witches, wizards and magic just like Harry Potter. Check it out. Oh, but I do warn you about one thing, once you start reading it, you might just crave a piece of chocolate!