In a saucepan over medium heat, combine diced Granny Smith apples, granulated sugar, ground cinnamon, cornstarch, and a splash of water. Stir gently and cook for about 5-7 minutes until the apples are softened and slightly caramelized, then remove from heat and let cool.

In a small bowl, mix dark brown sugar and a sprinkle of ground cinnamon for the topping. Set aside.

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

In a large mixing bowl, beat together unsalted butter and granulated sugar until light and fluffy, about 3-4 minutes. Add Greek yogurt and eggs, mixing well after each addition.

In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, kosher salt, and ground cinnamon. Gradually fold these dry ingredients into the wet mixture, alternating with a splash of additional yogurt until just combined.

Spread half of the batter in the prepared pan, layer the cooled apple mixture over the batter, and sprinkle two-thirds of the brown sugar-cinnamon mix on top.

Spoon the remaining batter over the apple layer and sprinkle the rest of the brown sugar-cinnamon mix on top.

Bake for 35-40 minutes, checking for doneness with a toothpick.

While the cake cools, prepare the glaze by whisking together powdered sugar and milk until smooth and pourable.

Once cooled for about 10 minutes, pour the glaze evenly over the cake and let set slightly before slicing.