0 Apple Filling: Peel and core the apples, then cut into thin ⅛” slices. Cut the slices in half to create smaller pieces.
1 In a medium saucepan, combine the apples, brown sugar, and cinnamon. Place the saucepan over medium-low heat and cook until the apples are tender and the liquid is bubbling, about 5-8 minutes.
2 Combine the water and cornstarch separately in a small bowl. Once the apples are soft and the mixture is bubbling, stir in the water and cornstarch mixture. Continue to stir while the mixture thickens and becomes glossy, about 30 seconds to a minute. Remove the apple filling from the heat and set aside to cool.
3 Preheat oven to 350°F. Lightly coat an 8” x 8” square metal baking pan with nonstick cooking spray and line with parchment paper to create a sling, with an overhang on all four sides.
4 Vanilla Cake: In the bowl of a stand mixer fitted with a paddle attachment, cream the butter and the sugar until fluffy and light in color.
5 Slowly add the egg and vanilla, with the mixer running on low speed. Scrape the bowl well with a rubber spatula, then mix on medium-high until the mixture is smooth and light in color.
6 In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Add the flour mixture and the sour cream to the bowl. Start the mixer on low speed and slowly stream in about half the milk. Mix just until combined, then scrape the sides and bottom of the bowl. Slowly stream in the remaining milk with the mixer running on low, until combined. Scrape the bowl again and give the mixture a few final folds with a spatula to ensure everything is incorporated.
7 Fold the slightly cooled apple filling evenly into the cake batter. Pour the batter into the prepared pan.
8 Cinnamon Sugar Topping: In a small bowl, combine the sugar and cinnamon. Sprinkle it evenly over the top of the cake batter. Bake the cake at 350°F for 30-35 minutes, or until a wooden skewer or toothpick comes out mostly clean.
9 Slice the cake, serve, and enjoy!
