Preheat oven to 350. Line the bottom of a 9" cake pan and grease the sides with nonstick spray.
In a large bowl, beat the butter and brown sugar with a hand mixer for 2 minutes or until light in colour and fluffy. Add apple sauce and beat again for 1 minute.
Add the eggs and beat until well incorporated. Add vanilla and beat once again.
In a separate bowl, whisk together dry ingredients.
Add dry ingredients to wet and mix until no dry ingredients remain. Add in milk and beat one final time until smooth.
In a small pan, add sugar, cinnamon, salt and water over medium-high heat. Stir to combine until it starts to bubble. Keep stirring and swirling the pan until the sugar mixture becomes amber in colour. Be careful not to overcook. My caramel took approximately 4 minutes.
Arrange apple slices as desired in the bottom of the prepared pan. Pour caramel over the apples.
Pour cake batter evenly over apple slices. Smooth out the top with a rubber or offset spatula.
Bake for 35-40 minutes or until a toothpick comes out clean.
Let the cake rest in the pan for 10-15 minutes and then flip it onto a plate.
Let the cake rest for 30 minutes before serving with cinnamon whipped cream.