In a medium saucepan, combine diced Granny Smith apples, white sugar, cornstarch, and ground cinnamon. Add a splash of water and cook over medium heat for 5–7 minutes until softened. Remove from heat and cool.

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

In a large mixing bowl, beat together room-temperature unsalted butter and granulated sugar until fluffy and light in color.

Add Greek yogurt, vanilla extract, and room temperature eggs to the creamed mixture. Mix until combined.

In a separate bowl, whisk together dry ingredients: all-purpose flour, baking powder, baking soda, kosher salt, and cinnamon. Gradually combine with wet ingredients.

Spread half the batter into the baking pan. Spoon the cooled apple mixture over, then top with remaining batter and sprinkle with cinnamon sugar.

Bake for 35–40 minutes until a toothpick comes out clean and the top is golden brown.

While cooling, make the glaze by whisking powdered sugar, milk, and vanilla extract together. Pour over warm cake after it has cooled slightly.