Preheat the oven to 350°F and line an 8x8 baking pan with parchment paper, covering all four sides of the pan
In a food processor or high speed blender, process the cashews and water until creamy, scraping the sides as needed. Make sure you end up with a very creamy paste and there are no chunks of cashews remaining.
In a bowl whisk the egg, then whisk in the cacao, flaxseed meal, honey, vanilla, baking soda and salt. Mix in the cashew butter until everything is well combined. Batter will be thick.
Pour batter evenly in the bottom of the prepared baking pan and bake until a toothpick inserted into the center comes out clean, approximately 20 minutes.
Set pan on a wire rack to cool then cut into squares.
Optional: melt about ¼ cup dark chocolate in a double boiler and drizzle over the top of each square. You can use a ziplock bag and cut a small whole on one end if you don't have a piping bag. Store in an airtight container at room temperature or refrigerated.