Preheat the oven to 350°F/180°c. Line an 8 inch square baking tin with parchment paper and set aside.
To make the crumb topping, add the flour, brown sugar and cinnamon to a large mixing bowl. Mix to combine, then add the softened butter. Use a fork to mash the butter into the other ingredients until well combined. Set aside.
To make the apple filling, add the diced and peeled apple, brown sugar, flour and cinnamon to another large mixing bowl. Mix until well combined and set aside.
To make the cake batter, add the softened butter and brown sugar to the bowl of a stand mixer. Beat on a medium-high speed using a paddle beater attachment for 3 minutes or until pale and fluffy. Add the eggs, sour cream and vanilla extract and beat until well combined.
Add the flour, baking powder, baking soda and salt and mix on a low speed until just combined.
Pour just over half of the batter into the prepared baking tin. Top evenly with the apple filling., followed by the remaining cake batter. Use your fingers to scatter over the crumble topping.
Bake the cake for 40-45 minutes, or until cooked through and a toothpick inserted into the center comes out clean or with a few moist crumbs. Allow the cake to cool in the tin for 10 minutes before transferring to a wire cooling rack to cool completely.
To make the vanilla glaze, add the powdered sugar and vanilla extract to a small mixing bowl along with 2 tablespoons of the milk. Mix until well combined, adding extra milk if necessary until the glaze reaches a pouring consistency. Drizzle over the cake before serving.