Preheat your oven to 350°F. Generously spray a 10 or 12-cup Bundt pan with nonstick cooking spray, making sure to coat every groove and the center tube.
In a large mixing bowl, combine the cake mix, eggs, vegetable oil, buttermilk, and vanilla extract. Beat with an electric mixer on medium speed for 2 minutes until smooth and slightly thickened. Set aside.
In a small bowl, stir together the brown sugar, flour, cinnamon, and melted butter until it resembles wet sand. This is your cinnamon swirl mixture.
Pour half of the cake batter into the prepared Bundt pan. Dollop half of the cinnamon mixture over the batter. Use a butter knife or skewer to gently swirl the cinnamon mixture through the batter in a marbling motion.
Pour the remaining cake batter into the pan. Add the rest of the cinnamon mixture on top and swirl again gently.
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter).
Remove from oven and let the cake cool in the pan for 15 minutes. Then carefully invert onto a wire rack or serving plate and allow to cool completely.
While the cake cools, make the glaze. Beat together the softened cream cheese and butter until completely smooth. Add vanilla extract and powdered sugar, mixing until glossy. If too thick, microwave for 15 seconds to thin.
In a small bowl, combine the sanding sugar and cinnamon for the topping.
Once the cake is completely cool, drizzle the cream cheese glaze over the top in a zigzag pattern. Immediately sprinkle the cinnamon sugar mixture over the wet glaze so it sticks.