I know that Margarine / Shortening are usually softer than butter, but what other differences are there? Can I just melt the butter and call it good or will this effect my final Product? I have seen Margarine / Shortening a lot in cookies, but I am looking for a general answer.
Here are the players:
- butter (~80% fat, salt varies)
- margarine (~80% fat, added salt)
- vegetable oil spread (less than 80% fat, salt varies)
- shortening (100% fat, no salt)
I only list it that way, because some people think a vegetable oil spread = margarine. It is not.
If you substitute an oil spread for butter, you could have problems.
My experience is that butter and true margarine can be substituted freely without negative results. Though, most people believe butter has a better flavor profile.
Salt content could also be a factor. Salt varies in different butter/margarine brands. I'm not sensitive to salt levels, but you might be.
The water content in butter/margarine can be enough to make things rise from steam action that you do not want risen (shortbread type doughs which you want to keep shape), or make things wet which you want to stay dry (molten chocolate)... and some textures might rely on the fat not melting below a certain temperature, or quickly going from solid to thin liquid and back as temperature increases/decreases.
And some types of shortening are harder/more brittle than butter/margarine (thinking of eg classic Palmin here).