Heat the oven to 350 degrees.

Place pistachio nutmeats in bowl of food processor. Using on/off turns, pulse the pistachios until they are crumb size. Do not overprocess, or you will have a nut butter.

Transfer the ground nuts to a medium bowl and add the sugar and salt. Stir well with a fork to combine. Add the vanilla to the egg whites and mix together. Pour the egg mixture over the nuts and distribute evenly, continuing to use a fork.

Transfer the wet nut mixture to a 9-inch tart pan. Press the nut mixture with a fork to spread it into a bottom crust and edges and come about halfway up the sides of the pan.

Bake until the crust is toasted lightly and is firm, about 15 minutes.

Meanwhile, combine the coconut milk, egg yolks, and chopped chocolate in a medium saucepan. Place over a low heat and stir constantly until the chocolate has melted. It will be steaming, but do not allow it to boil, as the mixture may separate.

Pour the chocolate mixture into the warm crust.

Let the tart stand at room temperature so that it will become firm. Cover the tart and refrigerate it for a couple of hours to increase the firmness.

Use a small sharp knife to loosen the crust from the sides of the tart pan.

Cut into wedges, garnish with additional pistachio nuts or berries, if desired, and serve.