Cook mandarins in a large pot of boiling water for three minutes. Drain and arrange slices in a single layer on paper towels.
Place butter in an 8-inch round cake pan. Mix vanilla seeds and xylitol and sprinkle over butter. Place in the oven until butter melts, about 7 minutes.
Whisk in the orange juice. Arrange mandarin slices over sugar in pan, starting in the center and spiraling outward.
In a large bowl, whisk together eggs, coconut milk, vanilla extract and sweetener.
In a smaller bowl, combine the almond and coconut flour, salt and baking soda.
Mix dry ingredients into the wet with a handheld mixer. Add a bit of stevia until the batter is slightly sweeter than you would want the cake to taste.
Spoon the batter into the cake pan on top of mandarins. Bake for 30 to 40 minutes.