Preheat your oven to 350° and spray your 9" cake pan with a non-stick cooking spray (I used coconut oil). Next pour in the 1/4 cup of melted butter and sprinkle in 2/3 cup light brown sugar.
Arrange the thin slices of clementines on top of the sugar.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt.
In a small bowl, rub the sugar with the zest until it resembles wet sand.
In a large mixing bowl, lightly whisk the eggs before adding in the zesty sugar, juice, oil, applesauce and vanilla. Then alternate adding 1/3 of the flour mixture with 1/2 of the buttermilk, beginning with and ending with the flour. Mix until just combined and then pour the batter overtop of the sliced clementines. Tap the pan on your counter to release any trapped air.
Slide the cake onto the middle rack of your preheated oven [place on the lower third rack if you have a tiny oven, like me!] and bake for 40 to 50 minute or until a cake tester comes out clean after inserted into he middle of the cake. While baking, if you notice that your cake is getting to brown, you can loosely cover the top with aluminum foil to keep the cake from getting too brown.
Then allow the cake to cool for 10 minutes before running a knife along the edge of the pan and inverting it onto a cake plate.
Once the cake has cooled, dust with powdered sugar before serving with whipped cream.
Store the cake in the fridge, wrapped in plastic wrap for 2 to 3 days.