Preheat the oven to 350 degrees. Line a 9 x 5 loaf pan with parchment paper and lightly grease it with non-stick cooking spray.

For the cake, to a mixer, fitted with the paddle attachment, cream together the unsalted butter, white sugar, and brown sugar on medium speed for 2-3 minutes or until the mixture is pale in color. Add the vanilla extract, and then one egg at a time, mixing well after each addition. Stir in the sour cream until fully incorporated.

In a medium bowl, whisk together the all-purpose flour, baking powder, cornstarch, salt, and cinnamon. Add half of the dry ingredients to the mixing bowl and mix until just combined. Pour in half of the milk, mix again, and then add the rest of the dry ingredients and milk. Mix on low speed for another minute until the batter is smooth.

In a separate medium bowl, prepare the filling by mixing together the flour, brown sugar, cinnamon, and unsalted butter using a fork or your fingers until crumbly.

Pour half of the cake batter into the prepared loaf pan. Sprinkle half of the filling evenly over the batter. Then add the remaining batter and top it off with the last of the filling.

Bake the cake for 55-60 minutes or until a toothpick inserted into the center comes out clean.

Let the cake rest in the pan on the counter for 20 minutes, then carefully remove it from the loaf pan. Allow it to cool completely before adding the glaze.

For the glaze, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until the glaze is smooth and free of lumps. Pour the glaze over the cooled loaf cake.