In a medium saucepan, combine diced Granny Smith apples, white sugar, cornstarch, ground cinnamon, and a splash of water. Cook over medium heat for about 5-7 minutes, until the apples soften and become fragrant. Once tender, remove from heat and allow to cool.

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

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

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

Add the applesauce, vanilla extract, and eggs to the creamed butter mixture. Blend until well combined and smooth.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, kosher salt, and ground cinnamon. Gradually incorporate into the wet ingredients, alternating with Greek yogurt.

Pour half of the batter into the prepared baking pan, spread evenly. Layer the cooled apple mixture on top, sprinkle half of the cinnamon sugar over it, then spread the remaining batter on top. Finish with the remaining cinnamon sugar.

Bake for 35-40 minutes. Check for doneness by inserting a toothpick into the center; it should come out clean.

Prepare the glaze by whisking together powdered sugar, milk, and vanilla extract in a bowl. Adjust thickness as desired.

Once baked, let the cake cool for a few minutes, then poke holes in the top with a toothpick. Drizzle the glaze over the warm cake.