How to Make It
Preheat oven to 350° F. Generously coat 20 cups of a miniature muffin pan with cooking spray.
Whisk egg whites in a large bowl until frothy, about 30 seconds. Add coconut, vanilla, 1 tablespoon of the maple syrup, and ¼ teaspoon of the ground sea salt, and whisk until well incorporated. Place about 2 teaspoons mixture in each muffin cup, pressing evenly with the back of a teaspoon into the bottom and up the sides of each cup. Bake in preheated oven until golden brown around the edges and dry in the center, about 15 minutes.
Meanwhile, microwave chocolate in a microwavable bowl on HIGH, until melted and smooth, about 1 minute, stopping and stirring every 30 seconds. Stir in cashew butter, cocoa, water, and remaining 1 tablespoon maple syrup and ¼ teaspoon ground sea salt until well incorporated. Transfer to a ziplock plastic bag, and snip a small hole in 1 corner. Pipe chocolate mixture into cooled tartlet shells. Sprinkle with flaky sea salt. Tartlets can be made up to 1 day ahead. Store, loosely covered, at room temperature.