I've been trying out the NFSC recipe for cookies, but because I'm vegan I've been replacing the butter with half vegetable shortening (Trex in the UK) and half vegan tub margarine (Vitalite). I tried replacing all the butter with Stork, which is the only vegan stick margarine I've come across in the UK, but I didn't like the resulting texture or taste. I've been chilling my cookies for 30 mins or so before baking them in order to try and stop them spreading, but they're doing it a bit and I assume this is because the tub margarine never sets up hard like butter.
Would freezing eradicate the spreading problem, do you think? Or would they still start spreading once the dough gets hot and "melts" the margaine?
If you freeze your cookies before baking them, how much extra do you allow for the baking time? I saw a recipe that said to freeze the cookies before baking them for 10-12 mins, but this is the time I normally bake unfrozen cookies for, so I'd be worried about the middles still being raw.
I don't know about vegan tub margarine, but traditional tub margarine is not recommended for baking because it has too much water. So if the vegan tub margarine also has too much water (which would explain the spreading), I would suggest adding extra flour to your batter to compensate for the additional liquid.
If you are trying different kinds of fat and it's still spreading too much, have you tried cutting the sugar? It's just that excess spreading is often a sugar problem.
Regular butter will turn to liquid when it is heated up in the oven so I personally kind of doubt that it's the margarine causing the problems. It's not like regular butter or margarine is going to stay "together" during baking any better.
I would also try the flour suggestion above It just sounds like there's something "off" in the formula that just needs tightening up.