In a medium saucepan over medium heat, combine the diced Granny Smith apples, white sugar, cornstarch, ground cinnamon, and a splash of water. Stir frequently for about 5-7 minutes until the apples soften and the mixture thickens slightly. Remove from heat and let it cool.

In a small bowl, mix together the ground cinnamon and white sugar for the topping. Set it aside.

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

In a large mixing bowl, beat the room-temperature unsalted butter and granulated sugar together until light and fluffy, about 3-5 minutes.

Mix in the applesauce, vanilla extract, and the eggs. Beat until fully combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and cinnamon. Gradually add this to the wet batter, alternating with Greek yogurt. Mix carefully until just combined.

Pour half of the batter into the prepared pan, then spread the cooled apple mixture evenly over it. Sprinkle half of the cinnamon sugar mixture on top, followed by the remaining batter and the rest of the cinnamon sugar.

Bake for 35-40 minutes. It's done when a toothpick inserted in the center comes out clean or with a few moist crumbs.

While the cake is baking, whisk together the powdered sugar, milk, and vanilla extract until smooth. Adjust milk for desired thickness.

Once the cake is out of the oven and slightly cooled, poke small holes across the surface with a toothpick. Pour the glaze over the top allowing it to soak in. Let it cool completely before cutting into squares to serve.