In a medium saucepan, combine diced Granny Smith apples, sugar, cornstarch, ground cinnamon, and water over medium heat. Stir occasionally for 5-7 minutes or until the apples soften and the mixture thickens.

In a small bowl, whisk together a mixture of cinnamon and sugar. Set it aside for later.

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

In a large mixing bowl, cream together the room-temperature unsalted butter and granulated sugar using an electric mixer on medium speed for 3-5 minutes until light and fluffy.

Add the applesauce, vanilla extract, and large eggs to the creamed mixture. Mix on low speed until combined.

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

Pour half of the batter into the prepared baking pan, spread it evenly, then spoon the cooled apple mixture over it, sprinkling it with the cinnamon sugar blend.

Pour the remaining batter over the top, spreading gently, and finish with any remaining cinnamon sugar.

Bake for 35-40 minutes. Check for doneness with a toothpick.

Prepare the glaze by whisking together powdered sugar, milk, and vanilla extract until smooth.

Once baked, drizzle the glaze over the warm cake, allowing it to soak in. Let it cool slightly before slicing.