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

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in the diced apples until evenly distributed throughout the batter.

Pour the batter into the prepared pan, spreading it evenly.

To prepare the crumb topping, combine the oats, brown sugar, and a sprinkle of cinnamon in a bowl. Cut in remaining butter until crumbly.

Sprinkle the crumb topping evenly over the cake batter in the pan.

Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted in the center comes out clean.

While the cake cools, prepare the cinnamon drizzle by combining powdered sugar, milk, and vanilla in a small bowl.

Once the cake has cooled, drizzle the cinnamon glaze generously over the top.

Allow the glaze to set for a few minutes before slicing and serving.