Preheat oven to 350° F, and apply cooking spray generously to 2 jumbo muffin tins (12 muffins total). Set aside.

Apple Topping: Peel two apples and slice into 16 slices each (32 slices total). In a small bowl, mix together brown sugar and cinnamon. Add the lemon juice and melted butter, then mix well. Toss the apple slices in the mixture until well coated, and then add 2-3 apple slices to each jumbo muffin tin and press into the bottom.

Oat Topping: mix together rolled oats, brown sugar, cinnamon, flour, and salt. Then mix in the melted butter. Evenly divide the oat topping into each of the muffin tins and lightly press down to make room for the cake batter.

Cake Batter: In a standing mixer, beat the butter and sugar on high until creamy (about 2-3 minutes). Scrape down the sides of the bowl and add the egg, vanilla extract, and buttermilk. Mix on medium speed until completely combined. Peel two apples and dice into small pieces (about ¼ inch cubes). Toss in a small bowl with one teaspoon of cinnamon and set aside. In a separate bowl, whisk together the flour, baking powder, baking soda, salt, one teaspoon of cinnamon, and cornstarch. Add the dry ingredients to the standing mixer and mix on lowest speed until there aren't any pockets of flour. Fold in the diced apples with a rubber spatula.

Bake: Evenly distribute the cake batter in the muffin tins. Smooth over the batter with a knife so that it's even. Bake for about 20 minutes. Once a toothpick comes out clean from the cake batter, they're done. Remove from oven and allow to cool for about 5 minutes in the tin.

Removing the Cakes: Slide a knife around the perimeter of the cake inside the tins to ensure no edges will stick. Place the cooling rack upside down on top of the muffin tins, and then flip both over together. Shake the tins a little to make sure all the cakes separate from the tins. If enough cooking spray was used, all of the cakes should come out together. If some apples are stuck to the bottom of the pan, just remove them with a fork and place back on the cakes.

Prepare the Glaze: mix the powdered sugar, vanilla extract, and heavy cream together in a small bowl. Drizzle the glaze over the cakes. Best served warm. Enjoy!