Preheat the oven to 350°F (175°C) and grease a 9×13 inch baking pan.

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

Add in the eggs, milk, and vanilla extract, mixing well to combine.

In another bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture to the wet mixture, stirring until just combined.

Gently fold in the diced apples and pour the batter into the prepared baking pan.

To prepare the crumb topping, combine the flour, brown sugar, and softened butter in a separate bowl; mix until crumbly. Sprinkle this mixture over the top of the batter.

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

While the cake cools, prepare the cinnamon drizzle by mixing together the powdered sugar, milk, and cinnamon until smooth. Drizzle this over the cooled cake just before serving.