Preheat oven to 350°F. Generously spray a 10-12 cup Bundt pan with nonstick cooking spray.
In a large bowl, combine vanilla cake mix, eggs, vegetable oil, buttermilk, and vanilla extract. Mix on medium speed until thick and smooth.
In a small bowl, mix brown sugar, flour, cinnamon, and melted butter until combined to make the cinnamon swirl.
Pour half the cake batter into the prepared pan. Spoon half the cinnamon swirl mixture over the batter. Use a toothpick or butter knife to swirl gently through the batter.
Repeat with remaining batter and cinnamon swirl mixture, creating a second layer of swirl.
Bake for 40-45 minutes until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool in pan for at least 15 minutes, then invert onto a serving plate. Cool completely before glazing.
For the glaze, beat cream cheese and softened butter until smooth. Add vanilla and powdered sugar, mixing until creamy. Add milk if too thick.
Transfer glaze to a piping bag and drizzle over cooled cake in a zigzag pattern.
Mix granulated sugar and cinnamon for topping. Sprinkle generously over the glaze before it sets.