Preheat oven to 350 degrees Fahrenheit. Butter and flour a 9" square brownie pan.
In a large bowl, stir together flour, baking soda, cinnamon, cloves, and salt. In another bowl, stir together applesauce, brown sugar, and cooking oil. Mix the wet ingredients into the dry ingredients and stir well.
Mix brown sugar, cinnamon, and cocoa powder (optional) together for the filling in a medium bowl.
Peel and core the apple and slice into thin slices. Arrange decoratively on the bottom of the pan. You can be as creative as you'd like with the pattern! Try to cover the whole surface.
Carefully spread half the batter into the pan and smooth out, being careful not to disturb the apples. This will be about a cup to a cup and a half of the batter.
Sprinkle the cinnamon sugar filling on top so it evenly covers the surface.
Pour in the other half of the batter and smooth it out, being careful not to disturb the cinnamon sugar filling. I found it easier to put the batter in little dollops across the top and then smooth it out by connecting the dollops together.
Gently swirl the cake batter and cinnamon sugar filling with a butter knife, being careful not to disturb the apples on the bottom. You just want to swirl it, rather than mixing it together, so I just did a couple S-shapes in each direction. Smooth the top of the batter again.
Bake for 35-40 minutes or until a toothpick comes out clean.
Flip the cake out onto a baking sheet so the apples are now on top, and replace any apple slices that may have come loose. Sprinkle another 1 tbsp brown sugar and 1/4 tsp cinnamon over the top of the cake and put under the broiler for 1-2 minutes or until the brown sugar melts onto the apples.
Serve with a nice cup of coffee for breakfast, or with whipped cream for dessert!