Place the can of coconut milk in the refrigerator and leave it there until well-chilled; I left mine in overnight.
Open the can of coconut milk. There will be a firm, waxy layer on top.
Scoop out this firm layer coconut cream that has solidified at the top of the can.
Stop as soon as you reach the water at the bottom of the can; don't include anything but the solid cream. (You can use the water in smoothies, or just drink it straight.)
Place this cream in the bowl of a stand mixer, or a large bowl.
Turn your mixer or hand beaters to high speed, and whip the coconut cream for 3 to 5 minutes.
Whip until it becomes fluffy and light, with soft peaks. Mix in sugar or vanilla, if using.