Combine the cake mix, eggs, sour cream, and oil in a large mixing bowl. Beat until well combined. Scrape down the sides and mix again.
Transfer half of the batter to the prepared pan and spread it into an even layer.
Mix the brown sugar and cinnamon in a small bowl. Sprinkle that mixture over the top of the batter.
Add the remaining cake batter on top of the brown sugar layer.
Bake for 30-35 minutes or until the center is set and no longer jiggly. You can also use the toothpick test to determine doneness.
Allow the cake to cool completely (at least 30-45 minutes) before adding the icing.
Prepare the icing by combining the confectioner’s sugar, vanilla, and milk in a small bowl. Whisk together until smooth.
Pour over the top of the cake and spread until even and smooth. Allow to cool for 25-30 minutes before serving.
Notes
Tips and tricks!Store in an airtight container at room temperature for up to 4 days.
The batter on top of the cinnamon sugar may be hard to spread.
The icing can be thinned with additional milk.