Line your cake tin (preferably one with a removable bottom) with a circle of baking paper, cut to fit. This will save you a lot of pain afterwards.
Add all the dry base ingredients to your blender and blend until fine. Then add the water if needed to make the mixture stick together. When you squeeze a bit between your fingers it should stick. Pour this mixture into your cake tin and flatten down completely with the palm of your hand (wet your palm to avoid it sticking), then place in the freezer.
Now add all your filling ingredients except the coconut oil and blend until super silky smooth. If you have a high-speed blender you'll get an even smoother result, but a food processor will also do just fine. Once blended, add your melted coconut oil and blend again until completely mixed. Taste and add more coconut nectar if you'd like it sweeter!
Pour the filling over the base, smooth down with a spatula, then pop back in the freezer and leave overnight to set.
In the morning, remove, top with the reserved zest and slice. Keep some in the fridge for the week, and put the remainder back in the freezer for dessert emergencies.