Preheat oven to 350 F, and grease or oil a 9 inch cake pan or 8x8 baking pan.
Making the bottom fruit layer first. Place 2 ½ tbsp melted butter or coconut oil in the bottom of pan and swirl around to cover the bottom of pan.
Sprinkle 2 tbsp of sweetener of choice all over melted butter or oil in pan.
Layer apple slices over the bottom of pan on top of the butter and sweetener. Sprinkle the apple slices with optional walnuts and ¼ tsp cinnamon. Set aside.
In a large mixing bowl combine all the ingredients for the apple cake: 3 eggs beaten, tbsp unsweetened coconut milk or almond milk, ⅓ cup unsweetened apple sauce, sweetener of choice: ⅓ cup pure maple syrup (paleo), or for low carb use ⅓ cup low carb maple syrup + 20 drops liquid stevia, 1 tsp GF vanilla extract, ½ tsp cinnamon, ½ tsp nutmeg, ¼ tsp sea salt, ½ tsp baking soda, 1 tsp apple cider vinegar, ½ cup coconut flour, sifted, adding coconut flour last.
Stir vigorously while scraping the sides of bowl. Be sure to break up any coconut flour clumps.
Once thoroughly combined, pour or spoon cake batter over the apple fruit layer in the pan. Spread cake batter around evenly.
Bake at 350 for 35 to 40 minutes, or until center of cake is done.
Remove from oven and cool.
Can make optional caramel sauce while cake cools, or if skipping caramel sauce then can turn cake upside down on a plate once cooled.
In a small sauce pan over medium high heat combine and melt: 2 tbsp butter or coconut oil, 3 tbsp heavy cream or coconut milk, and 2 tbsp sweetener of choice (do not add vanilla extract yet).
Stir continuously while heating to a low boil. Once at a low boil remove from heat. Add and stir in the ¼ tsp vanilla extract.
Use a butter knife between cake and side of pan to slide around and loosen cake.Take a large plate and put over the top of cake pan, and turn plate and cake pan over. Making cake upside down with fruit now on top of cake.
Poke holes all over top of cake with metal or bamboo skewer.
Pour or spoon caramel sauce over top of cake.
Slice and serve. Store unused portions in the fridge.