Willow is 10 weeks old now and had decided our cat ( Minstrel) is fair game. The cat has his own puppy proof part of the house, but when he's outside Willow chases him when she sees him. Unfortunately the cat runs away, which of course is great fun.
I took Willow round to a friend's house the other day and her cat stood his ground and Willow then gave him a 'I'd never dream of chasing you - there must be some misunderstanding' type look.
Minstrel was great pals with our old dog, Thistle, who died in June, but he's scared of the puppy.

Cats outside are fair game to chase unfortunately even our own cats, if they move fast or walk on a wall, fence they will be chased. Most of our dogs will cuddle up to our cats in the house, eat next to them and sleep next to them, and most of our cats will learn if they walk slowly a dog won't chase.

However, if a dog won't learn a cat will give them a good old slap which will certainly teach them a lesson.

It's instinct and you can't stop that, but what you can do is train commands 'leave, stop, sit, away' I start training at 8 weeks, making it fun and rewarding, get a whistle or clicker and start already if you haven't, along with that when your pup starts chasing your cat distract turn his attention away and on you if you can to play instead.

But, don't be too hard on him, it really is a natural instinct dogs are predators (just like cats) you can't turn that off, it's always there. :-)

Mine chase our cats on occasion unfortunately have not trainined it out of them  Yes the cats do run as well which dosent help. Just watch pups eyes from amy cat swipes. My little nearlylost and eye from a cat swipe when she was around 14 weeks it perforated her eye ball. Fortnuately our wonderful vet saved the eye and she seems to have perfect sight. The cat wasnt ours she was in a friends house at the time and got to ear their cat

> However, if a dog won't learn a cat will give them a good old slap which will certainly teach them a lesson.

Not necessarily.  Remy has chased cats from day 1, despite growing up with 3, and has been swiped many times - when he was 5 months old a cat refused to run away, stood his ground and left a neat line of punctures right on his nose and he didn't so much as blink!

You need to physically prevent him chasing as well as training a rock solid 'leave' - keep him on a houseline for the time being if need be, make sure the cats aren't in the garden before you let him out or take him out on lead.  The biggest mistake I made with Remy was not doing that -  he chased because he had ample opportunity to and I failed to stop him having those opportunities.

Set him up to succeed - if he CANNOT chase the cats then you will have plenty of chances to instigate and reward non-chasing behaviour.