Preheat the oven to 180°C/Mark 4 and lightly grease two 20cm sandwich tins with sunflower oil. Line the bottoms with parchment paper.
Mix flax seeds with warm water to make your flax egg, then set aside for 10 minutes to thicken.
In an electric stand mixer with a paddle attachment, combine the dry ingredients.
In another bowl, combine the wet ingredients, including the flax egg. Then add the wet ingredients to the dry ingredients and mix for 2 minutes to make sure they are well incorporated.
Let them cool in the pan for 10 minutes before gently unmolding them onto a wire rack to cool completely.
For the buttercream frosting, drain and rinse the cashews, discarding the soaking water. Place them in the blender with all the other ingredients and blend until the frosting is creamy and smooth.
Spread the frosting on top of the first cake, then place the second cake on top like a sandwich and cover the top with the remaining frosting.
Decorate your cake with the walnut halves sprinkled around the edges and the orange zest generously sprinkled on top.