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

In a small bowl, mix together ground cinnamon and white sugar until well combined; set aside.

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

In a large mixing bowl, using an electric mixer, beat together softened unsalted butter and dark brown sugar until light and fluffy, about 3-5 minutes.

Blend in Greek yogurt, vanilla extract, and large eggs into the butter-sugar mixture, beating until smoothly combined.

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

Pour half of the batter into the prepared baking pan, spreading it evenly. Spoon the cooled apple filling over this layer, sprinkle with half of the cinnamon sugar, then dollop the remaining batter and finish with the rest of the cinnamon sugar.

Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean. The top should be golden brown.

Prepare the glaze by whisking powdered sugar, milk, and vanilla extract in a medium bowl until smooth. Adjust consistency as needed.

Once the cake has cooled, poke holes in the surface and drizzle the glaze evenly over the top.