Begin by preheating your oven to 325 degrees Farenheit.
Combine the cake mix, oil, eggs, and sour cream in a large mixing bowl. Mix by hand until most clumps are gone and batter is smooth. Pour half of the batter into a 9×13-inch baking dish.
In a separate bowl, mix the brown sugar and cinnamon. Sprinkle the cinnamon sugar mixture over the batter in the cake pan.
Then, pour the remaining batter over the cinnamon sugar mixture into the cake pan.
Swirl the batter with a knife, but do not mix.
Place the dish in the oven for 40 minutes, or until a toothpick comes out mostly clean.
While the cake is baking, prepare the frosting. Use a small bowl to which together the confectioners’ sugar, milk, and vanilla until it is a frosting-like consistency.
Once the cake is done baking, frost the cake while it is still warm.
