Preheat the oven to 325 degrees. Generously butter a 9 x 13 pan or glass baking dish and set aside.
In a small bowl, combine the brown sugar and cinnamon. Set aside.
In a medium mixing bowl, combine the cake mix, canola oil, eggs, and sour cream. Whisk or beat with a wooden spoon until thoroughly combined. Spread half the batter in the bottom of the prepared pan. Sprinkle the brown sugar and cinnamon evenly over the batter. Top with remaining cake batter. Use a knife to make swirls through the cake batter.
Bake for 35-40 minutes or until a tester inserted in center of cake comes out clean. Remove from the oven and place on a cooling rack. Let cool for 15-20 minutes.
In a small bowl, combine the sifted powdered sugar, milk, and vanilla extract. Blend until well mixed. While cake is still warm (but not hot), spread the glaze evenly over the top. Allow to cool to room temperature before serving.