In a medium saucepan, combine the sugar, salt, and cornstarch. Whisk in the coconut milk and set over medium-high heat. Whisk continuously until the mixture starts to simmer. Immediately turn the heat to medium-low and continue to whisk continuously until the mixture begins to thicken, 4 to 6 minutes. The mixture will go from liquid to very liquid before it starts to thicken. Pour in the whole milk in a steady stream while continuing to whisk. Bring the heat back up to medium and continue whisking until the mixture has thickened to the consistency of a thick glue and can coat the back of a spoon, another 4 to 6 minutes. Remove from the heat, pour into an 8-inch square baking pan, and smooth the top with a rubber spatula. Let cool for 10 minutes before covering the top of the pan with a sheet of plastic wrap. Transfer the pan to the refrigerator to chill and set for 2 hours. For an extra-smooth top, carefully place a sheet of plastic wrap directly on the top of the haupia after it has cooled for 10 minutes. Remove the plastic wrap and, using a sharp knife, cut five columns and five rows to make 25 squares. Place each piece in a paper muffin-tin liner and serve chilled. If desired, sprinkle the toasted coconut flakes on top, right before serving.