Preheat the oven to 350°F. Grease and flour a 9×13-inch baking dish.
In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Add the milk, eggs, vanilla extract, and melted butter. Mix until just combined. Fold in the diced apples.
Mix your brown sugar with cinnamon together in a small bowl.
Pour half of the batter into the prepared baking dish. Sprinkle half of the brown sugar and cinnamon mixture evenly over the batter. Repeat with the remaining batter and topping. Swirl gently with a knife.
Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean.
While the cake bakes, whisk together the powdered sugar, milk, and vanilla extract to make the glaze.
Let the cake cool for 10 minutes, then drizzle the glaze over the warm cake. Slice and serve.