Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.

In a medium bowl, whisk together 2 cups of flour, baking powder, baking soda, salt, and 1 teaspoon of cinnamon. Set aside.

In a large 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.

Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk. Mix until just combined. Fold in the diced apples.

Pour the batter into the prepared baking pan and spread it evenly.

For the crumb topping, in a small bowl, combine ¾ cup flour, brown sugar, and ½ teaspoon cinnamon. Stir in the melted butter until the mixture resembles coarse crumbs. Sprinkle the crumb topping evenly over the batter.

Bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean.

While the cake is baking, prepare the cinnamon drizzle. In a small bowl, whisk together powdered sugar, milk, and ½ teaspoon cinnamon until smooth.

Once the cake is done, allow it to cool in the pan for about 10 minutes before drizzling with the cinnamon glaze. Serve warm or at room temperature.