Preheat oven to 350°F. Grease a 9x13-inch baking dish with non-stick cooking spray.
Reserve ½ cup of the dry cake mix; set aside.
In a large mixing bowl, combine the remaining cake mix, butter, eggs, and sour cream. Mix just until smooth.
Spread half of the batter into the prepared baking dish.
Mix the reserved cake mix, brown sugar, pecans (if using), and 1 teaspoon cinnamon. Sprinkle over the batter.
Dollop the remaining batter over the filling and gently spread to cover.
Bake for 40 minutes, or until a toothpick inserted in the center comes out clean.
For the frosting, mix powdered sugar, the remaining 1 teaspoon cinnamon, and 3 tablespoons heavy cream until smooth. Spread over the warm cake.
Serve slightly warm or at room temperature.