In a medium saucepan, combine diced Granny Smith apples with white sugar, cornstarch, ground cinnamon, and a splash of water. Cook over medium heat for about 5-7 minutes, stirring often, until the apples are soft and the mixture has thickened. Remove from heat and set aside to cool.

In a small bowl, mix together the ground cinnamon and white sugar. Set this mix aside as you prepare the cake batter.

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

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

Add the applesauce, vanilla extract, and room temperature eggs into the creamed butter and sugar. Mix until smooth.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, kosher salt, and ground cinnamon. Gradually mix this into the wet batter, alternating with Greek yogurt until just combined.

Pour half of the cake batter into the greased pan, layer with cooled apple mixture, and sprinkle half of the cinnamon sugar on top. Spread remaining batter and sprinkle with rest of cinnamon sugar.

Bake in the preheated oven for 35-40 minutes, until a toothpick inserted comes out clean. The cake should be lightly golden.

Prepare the glaze by whisking together powdered sugar, milk, and vanilla extract. Adjust consistency as needed.

Allow the cake to cool slightly. Poke holes and pour the glaze over the warm cake. Let cool before slicing and serving.