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

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, cream the softened butter and granulated sugar until light and fluffy, then beat in the eggs one at a time, followed by the vanilla.

Gradually add the flour mixture to the creamed mixture, alternating with the chopped apples, and mix until just combined.

In a separate bowl, combine rolled oats, brown sugar, ground cinnamon, and softened butter to create the crumb topping.

Pour the batter into the prepared baking pan and sprinkle the crumb topping evenly over the surface.

Bake in the preheated oven for 40-45 minutes or until a toothpick inserted into the center comes out clean.

While the cake cools, prepare the cinnamon drizzle by whisking powdered sugar, ground cinnamon, and enough milk to reach your desired consistency.

Once the cake has cooled, drizzle the cinnamon mixture over the top and serve warm or at room temperature.