In a medium saucepan over medium heat, combine diced Granny Smith apples, sugar, cornstarch, ground cinnamon, and a splash of water. Stir gently and cook for about 5-7 minutes until the apples are softened and the mixture is bubbly. Remove from heat and allow to cool slightly.

In a small bowl, mix together the remaining ground cinnamon and sugar until well combined. Set this cinnamon sugar mixture aside.

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan.

In a large mixing bowl, beat together softened unsalted butter and white sugar using an electric mixer on medium speed until light and fluffy, around 3-5 minutes.

Add in the applesauce, room temperature eggs, and vanilla extract to the creamed butter and sugar mixture. Blend until smooth and well combined.

In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, kosher salt, and a touch of cinnamon. Gradually add these dry ingredients to the wet mixture, alternating with the Greek yogurt. Mix just until combined.

Pour half of the batter into your prepared baking pan, spreading it evenly. Spoon the cooled apple filling over the batter and sprinkle half of the cinnamon sugar mixture. Pour the remaining batter on top and sprinkle the rest of the cinnamon sugar.

Place the baking pan in the preheated oven and bake for 35-40 minutes, checking doneness with a toothpick.

While the cake is baking, prepare the glaze by whisking together powdered sugar, milk, and vanilla extract until smooth.

Once the cake is slightly cooled, poke holes all over the top and pour the prepared glaze over it.