1. Put the palm sugar, lemon juice, and ¼ cup water into a small saucepan, set over mediumhigh heat, and cook, stirring, to dissolve the sugar if necessary until the mixture becomes dark brown and registers 350°F on a candy thermometer, about 12 minutes. Remove from the heat and stir in ¼ teaspoon of the salt. Carefully divide the caramel among twelve 4-ounce ramekins.
2. Put the milk, coconut milk, coconut, sugar, and the remaining ½ teaspoon salt into a medium saucepan, set over medium heat, and warm, stirring occasionally, until bubbles begin to form around the edges. Remove from the heat and let steep for 30 minutes.
3. Preheat the oven to 350°F.
4. Add the evaporated milk to the coconut mixture.
5. Whisk the eggs and egg yolks in a large mixing bowl until the yolks are broken, then add the milk mixture in a slow, steady stream, whisking constantly. Strain the mixture through a fine-mesh sieve into a bowl, pressing on the coconut to extract as much liquid as possible. Set the mixture in a larger bowl filled with ice and water and chill completely until cold.
6. Divide the coconut mixture among the prepared ramekins and put in a deep baking dish or roasting pan. Carefully pour enough hot water into the pan, without spilling any into the ramekins, to reach halfway up the ramekins. Bake until the Han is just set, about 45 minutes—the sides should be firm but the center still quite jiggly. Cool completely in the pan.
7. Serve the flan once it has cooled or, for even better results, cover and refrigerate for at least 4 hours. Invert the flans onto serving plates. If the flan seems stuck, run a thin-bladed knife around the edges of each ramekin, and then invert. Sprinkle each flan with fleur de sel, if desired.