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

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

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

In a large mixing bowl, beat the softened unsalted butter and granulated white sugar together using a hand mixer on medium speed for about 3-5 minutes until light and fluffy.

Incorporate the applesauce, vanilla extract, and large eggs into the butter-sugar mixture, mixing until smooth.

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

Pour half of the prepared batter into the greased baking pan, spreading it evenly. Spoon the cooled apple mixture over the batter and sprinkle half of the cinnamon sugar blend on top. Finish by pouring the remaining batter over the apple layer and sprinkling the rest of the cinnamon sugar.

Place the baking pan in the preheated oven and bake for 35-40 minutes until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for about 10 minutes before transferring to a wire rack.

In a mixing bowl, whisk together powdered sugar, milk, and vanilla extract until smooth and shiny. Adjust the milk to reach desired thickness.

Poke small holes in the slightly cooled cake to allow the glaze to seep in. Pour the glaze evenly over the warm cake and let it soak in before slicing and serving.