Prepare the oven and pan. Gather all of the ingredients. Heat the oven to 325 degrees F (165 degrees C), then lightly grease a 9×13-inch baking pan.
Mix the cake batter. Place the yellow cake mix, sour cream, eggs, and vegetable oil in a bowl. Beat them together with an electric mixer for 3 minutes.
Add the first batter layer. Pour 1/2 batter into the prepared baking pan and spread it across the base.
Make the cinnamon filling. Stir the brown sugar and ground cinnamon together in a small bowl, then sprinkle the mixture over the batter already in the pan.
Cover the filling. Carefully pour remaining 1/2 batter over the brown sugar-cinnamon layer, keeping the filling between the two portions of cake batter.
Bake until the centre is done. Place the cake in the preheated oven and bake for 45 to 50 minutes. Insert a toothpick into the centre to check it. The toothpick should come out clean.
Mix the glaze. Combine the confectioners’ sugar, milk, and vanilla extract in a bowl.
Glaze the hot cake. Use a fork to poke holes into the cake while it is still hot, then pour the confectioners’ sugar mixture over the top.
Let the cake cool. Allow the finished cake to cool completely before serving, about 30 minutes.