Even though it isn't really milk (in the dairy sense), coconut milk still naturally separates into a thick cream and thinner liquid like regular milk. As such, when working with coconut milk you should still follow the same procedures you would to make a milk-based cream sauce.
The number one rule when making any creamy sauce is: DON'T LET IT BOIL! Boiling will guarantee that your creamy sauce (including sauces made with coconut milk) will break in some form or fashion. At most, you should cook these at a bare simmer.
Other than that, there are some techniques you can use to keep your curry smooth.
You could use an emulsifier like honey (common in vinaigrettes, where it is used to make sure the oil and vinegar don't separate), added toward the end of cooking.
You could also use a thickening agent, like a cornstarch slurry or a quick roux. Curry paste is also a thickening agent. As a general rule of thumb, when making Thai-style curry I usually cook my vegetables in a little more oil than I think they need, then add the curry paste and sauté that until it has absorbed the oil (along with any dry spices). It will act as a roux for the coconut milk and make sure there are no lumps in the final curry.
Lastly, cooking the curry uncovered at a simmer, stirring occasionally, will thicken it up nicely and help all the ingredients stay together.
- Empty the water from inside the coconut, as this is not the milk you want.
- Crack open your coconut and scrape out the meat.
- In a blender, take about half your meat and roughly 1 1/2 cups of hot water (more or less depending on your desired thickness), and process.
- Repeat step 3 with the other half of the meat.
- Place a few layers of cheesecloth over a bowl and strain the milk through.
- Wrap up the cheesecloth and squeeze the rest of the milk out.
No special press or tools needed. However, some sort of press could help get the last few drops out when squeezing the mass through the cheesecloth.
I don't think the water necessarily has to be hot. Some sources say to just use water, while other say hot. You may also want to start out with just 1/2 cup water and see how much it yields. Keep in mind: coconuts aren't super juicy. So expect the yield to be roughly equivalent to the amount of water used.
The flesh is still perfectly usable, but its applications may be limited based on how much you processed it (in terms of size).
This site has the step-by-step I used with a few (albeit not very helpful) pictures.
Best Answer
Half and half or heavy cream is a pretty good substitute. It has similar fat/water suspension, reduces and thickens similarly, and emulsifies similarly. You lose the coconut flavor, but it cooks similarly and you keep the texture.
You cannot substitute coconut cream, because it is too rich, which changes the texture of the sauce greatly, and doesn't do as good a job of getting flavors out of spices.
Incidentally, I highly suggest you pick up a box of instant coconut milk powder. It's not as good as the canned stuff (needs whisking), but because it's nonperishable you don't have to use the whole container at a time. This helps avoid the "oh crap, used the last can!" moments you get with canned coconut milk.