Preheat & Prep Pan: Preheat oven to 350°F (175°C). Grease a 9x13 inch baking pan generously with butter or non stick spray. Set aside.
Prep Apples: In a small bowl, toss the diced apples with 2 tablespoons cornstarch until evenly coated. Set aside.
Make Cake Batter: In a large bowl, whisk together flour, granulated sugar, baking powder, and salt. Add milk, eggs, and vanilla. Beat with an electric mixer until smooth.
Finish Batter: Stir in the 4 tablespoons of melted butter. Gently fold in the cornstarch coated apples using a rubber spatula.
Pour Batter: Pour the batter into the prepared pan and spread into an even layer.
Make Cinnamon Swirl: In a separate medium bowl, beat the softened butter, brown sugar, and cinnamon together until light, fluffy, and well combined.
Swirl: Drop spoonfuls of the cinnamon mixture randomly over the cake batter. Gently swirl with a knife. Do not over mix.
Bake: Bake for 55-65 minutes, or until a toothpick inserted in the center comes out clean.
Cool: Place the pan on a wire rack and cool for 15-20 minutes (cake should be warm, not hot).
Make Glaze: In a small bowl, whisk powdered sugar, 4 teaspoons of milk, and vanilla. Add the 5th teaspoon of milk if needed for a pourable consistency.
Glaze & Serve: Drizzle glaze over the warm cake. Let set for 10 minutes before slicing.