Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish with non-stick spray.

In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. Set aside.

In a large bowl, use an electric mixer to beat the softened butter and sugar until light and fluffy (about 2 minutes). Beat in the eggs one at a time, followed by the vanilla extract.

Add half of the dry ingredients to the butter mixture, mixing just until combined. Add the sour cream, mix, and then add the remaining dry ingredients. Do not overmix.

Gently fold the diced apples into the batter using a rubber spatula.

Spread the batter evenly into the prepared baking dish.

In a small bowl, prepare the streusel topping by cutting cold butter into a mixture of brown sugar, flour, and cinnamon until it resembles coarse crumbs. Sprinkle this evenly over the batter.

Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool completely before drizzling with vanilla glaze and slicing into squares.