Get an 8x8 inch pan.
Place coconut milk in a small saucepan over medium heat.
Meanwhile, combine sugar and cornstarch in a bowl, and add the water. Whisk until completely combined.
When the coconut milk just starts to simmer, add the sugar/cornstarch mix, and whisk. Keep whisking until the mixture thickens and starts looking slightly translucent. This took me about 10 minutes. When you bring your whisk up, the haupia should flow off of it and you can see the trace of it for a few seconds before it "melts away".
Pour into the 8x8 pan and allow to cool on the counter before refrigerating. Refrigerate until solid and completely cooled. If you are using the full 5 tablespoons, you should be able to pull the sides away from the pan and invert the whole thing onto a cutting board. If it seems too soft for that, just cut it while in the pan. Cut into 2 inch pieces (16 pieces total) and serve.