In a medium saucepan, combine the diced apples, granulated sugar, cinnamon, cornstarch, and water. Cook over medium heat for 5 to 6 minutes, stirring frequently, until the apples soften and the mixture thickens. Remove from heat and allow to cool slightly. In a separate small bowl, mix the dark brown sugar with the remaining 1/2 teaspoon cinnamon. Set aside.

Preheat the oven to 350°F (175°C). Lightly grease a 9×13-inch baking pan with nonstick spray and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and granulated sugar on medium speed for approximately 3 minutes, or until light and fluffy. Add the applesauce, vanilla extract, and eggs, mixing until fully incorporated and scraping down the sides of the bowl as needed.

In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Add the dry ingredients to the batter in alternating additions with the Greek yogurt, beginning and ending with the dry ingredients. Mix just until combined.

Spread half of the batter evenly into the prepared baking pan. Spoon the apple filling over the batter, spreading carefully to cover the surface. Sprinkle approximately two-thirds of the brown sugar mixture evenly over the apples.

Dollop and gently spread the remaining batter over the filling. Sprinkle the remaining brown sugar mixture evenly over the top.

Bake for 35 to 40 minutes, or until the center of the cake is set and a toothpick inserted into the center comes out mostly clean.

During the final minutes of baking, whisk together the powdered sugar, milk, and vanilla extract until smooth.

Once the cake is removed from the oven, immediately use a butter knife to poke approximately 30 holes evenly across the surface of the cake. Pour the glaze over the hot cake, spreading evenly to ensure it seeps into the holes. Allow the glaze to set for about 20 minutes before slicing and serving.