Preheat the oven to 325 degrees F (165 degrees C).

In a large mixing bowl, combine cake mix, oil, eggs, and sour cream. Stir by hand for approximately 50 strokes, or until most large lumps are gone. Pour 1/2 of the batter into an ungreased 9x13-inch glass baking dish. Combine brown sugar and cinnamon; sprinkle over batter in the cake pan. Spoon the remaining 1/2 of the batter into the cake pan, covering brown sugar and cinnamon. Swirl batter with a butter knife or icing knife until it looks like a honey bun (or whatever design you want to make).

Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Frost cake while it is still fairly hot. Serve warm.

To make frosting: In a small bowl, whisk together the confectioners' sugar, milk, and vanilla until smooth.