What is the purpose of cream of tartar in hard English toffee?

And why is the JoC recipe for English Toffee so drastically
different from other toffee recipes? It says to use a large amount
of cream and less butter, and less sugar and only take the mixture
to 270F.

While a "real" English Toffee recipe is about equal weight sugar
and butter, with a little water and corn syrup and is cooked to
300-310F. This produces the "Heath" bar type toffee that I think
most of associate with toffee.

FWIW, I just made my first successful batch of toffee in a long
time. I used to make it perfectly all the time. Then 5 in a row
failed over the last 2 years. I guess I got my groove back.

I used a non-stick All Clad Ltd high-sided sauce pan (I can't find
a picture of this pan anywhere on the web - it must be pretty rare-
not part of any set).

I think the non-stick helped the mixture from recrystalizing. I
also did the pasty brush thing, but it seemed unnecessary. When I
would use a bare SS pan the bubbles would stick to the sides. But
not this time.

-sw