Preheat your oven to 350°F. Grease and flour a 9-inch round or square baking pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream, beginning and ending with the dry ingredients. Mix until just combined. Do not overmix.
Gently fold in the Brainiac Apple Cinnamon applesauce until evenly distributed throughout the batter. Pour the batter into the prepared baking pan, spreading it out evenly.
In a small bowl, prepare the streusel topping by combining the flour, brown sugar, and ground cinnamon. Stir in the melted butter until the mixture resembles coarse crumbs. Sprinkle the streusel evenly over the cake batter.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Once the cake is done, remove it from the oven and let it cool in the pan for 10 minutes. Then, transfer it to a wire rack to cool completely.
While the cake is baking, prepare the glaze by whisking together the powdered sugar, milk, and vanilla extract until smooth. Adjust the consistency by adding more milk if too thick or more powdered sugar if too thin.
Drizzle the glaze over the cooled cake. Slice and serve the Brainiac Apple Cinnamon Coffee Cake as a delightful treat for breakfast or dessert. Enjoy!