YES!!! is the short answer.

Chocolate contains a lethal ingredient (to cats), a methylxanthine
called theobromine, and baking chocolate contains 10 times more of this
lethal ingredient than milk chocolate.

Theobromine acts on four areas of your cat's body:

It increases the rate and force of contractions of the heart.
(Tachycardia)
It acts as a diuretic, causing your pet to lose body fluids. (via
Urine)
It affects the gastrointestinal system, causing vomiting and diarrhoea
and it may cause stomach ulcers.
It acts on the nervous system, causing convulsions, seizures and
sometimes, death.

If you find that your cat has ingested some chocolate, call your vet
immediately. He will probably ask you the size of your cat, the type
and quantity of chocolate your cat has eaten, and how long ago it was
eaten. Try to have these answers before you call. He then may tell you
to make your cat vomit. This will depend on the amount of chocolate
ingested and how long ago it was eaten. If your decides that your cat
needs to come into the surgery, do not delay. The effects of chocolate
toxicity may not be apparent right away, but do not let that lull you
into a false sense of security.

Having said that, my cat doesn't appear to be interested in chocolate,
unlike me. I never met a chocolate that I didn't like!

I hope I have averted a disaster, Phew! and all because the lady loves
Milk Tray! LOL

Good luck!