Bring the ingredients for the two layers to boil in two separate pots. Wash the millet before boiling. Personally I tend to add the xylitol and coconut sugar during the blender, but you can also choose to boil them with the other ingredients.
Boil until the millet is soft and like porridge. Be careful that your millet isn't too dry and burns. In that case, add some milk or water.
First blend one of the layers to a smooth mass. Taste to check if it needs any extra sweeteners or other ingredients. Your mixture should be liquid enough to pour it into the spring form but not so liquid that it won't stiffen up in the fridge. Keep the mixture of the first layer in a container that's easy to pour it out off later.
After cleaning your blender, blend the other layer. The same advice goes for this layer.
Now the magic. Pour some of the white mixture onto your base, more or less the amount to make it touch the edges of the cake form. Then pour some of the black mixture right in the middle of the white mixture, and let gravity do its work so that it floats a little towards the sides. Then pour some of the white mixture right in the middle, then black, until everything's finished or the spring form is full. I had enough to make two extra little cakes. Your cake is now ready. Put it in the fridge.