Heat oven to 350 F, 180 C, gas 5. Oil two 23-cm sandwich cake tins.
Mix flour, cacao, baking powder, baking soda and sea salt together in a large bowl.
Place all the other ingredients in a blender or food processor and process until well mixed. Add this mixture to the dry mixture and fold lightly together.
Put half the mix into each pan and bake for 40 - 50 mins. A toothpick should come clean when it is done.
Refrigerate the cans of coconut milk overnight and then separate the solid part from the liquid part. Using only the solid part, combining it with the chilled maple syrup and mix until firm, adding a pinch of sea salt.
When the cake layers have completely cooled join them together using the coconut frosting. The top can be dusted with arrowroot powder.