Combine the cake mix, oil, eggs, buttermilk and sugar. Beat for two minutes on medium speed. Pour half the batter into a greased 13-by-9-by-2-inch baking dish.
Combine 2/3 cup firmly packed brown sugar and 1 tablespoon ground cinnamon. Sprinkle half of this over the cake batter. Spoon the remaining batter over the brown sugar mixture. Using a knife, swirl through the cake batter. Bake at 350 degrees until lightly browned and a toothpick comes out clean.
Drizzle the warm cake with a mixture of 3/4 cup powdered sugar and 2 tablespoons of lemon juice.