In Brampton, at the little plaza at Peter Robertson and Torbram, is a small Indian restaurant called Sweet Palace. You'll walk in and see your usual fare of sweets. Walk to the cash, and you'll see a vegetarian thali bar. Look up, and you'll see a menu of items that can be made to order, along with several non-veg items.
I'm Indian, and I've been getting my butter chicken fix here. Now, mind you, I live in Brampton, so I might just be lazy and not tried other places to compare. But I love the chicken items here. They're all good, but you asked for butter chicken.