Remove can of coconut milk from the refrigerator and flip it upside down (this will cause all the liquid to be at the top). Open the can, and pour the coconut water into a separate bowl (you can save this for smoothies or discard it).
You will be left with the coconut cream in the bottom of the can - this is the thick, white creamy part. Scoop this out and transfer to another large bowl. With a hand mixer or whisk, whip the coconut cream until fluffy. Add the maple syrup and vanilla and whip again until combined. Place in the fridge until ready to assemble the tart.
*For a great description of how to pick the right coconut milk and how to whip it properly, check out this tutorial at Oh She Glows: http://ohsheglows.com/2012/08/30/coconut-whipped-cream-a-step-by-step-photo-tutorial/