Blitz cashews, buckwheat and dates in a food processor for 10-15 seconds, until broken down and starting to stick together. Line the base of your tart tins (or tart tin, if making 1 large tart) and pop the bases in the freezer to chill while you make the mousse.
Next, add avocado, water + coconut water powder, cacao powder, maple syrup, orange juice, vanilla and salt to a high-powered blender and blend until combined. Add a small amount of additional water if mix is too thick and becomes troublesome to blend.
Remove your bases from the freezer and pour the avocado on top, filling it just under the top of the tin rounds. Place in the freezer for around 2-3 hours (slightly longer if making 1 large tart).
Remove from freezer and plate. I serve mine with a sprinkle of beetroot powder over the top.