Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking pan.
In a large bowl, cream together the softened butter and sugar until the mixture is fluffy, about 3-4 minutes, then add the eggs one at a time, mixing well after each addition, and stir in the milk and vanilla extract until combined.
In a separate bowl, whisk together the flour, baking powder, cinnamon, and salt, then gradually add the dry ingredients to the wet mixture, mixing until just combined, and gently fold in the diced apples.
In another bowl, combine the brown sugar, oats, flour, melted butter, and cinnamon until it resembles coarse crumbs.
Pour the cake batter into the prepared baking pan, spreading it evenly, and sprinkle the crumble topping over the batter, ensuring an even layer.
Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean, and allow the cake to cool slightly before serving.