Preheat your oven to 350°F (175°C). In a large bowl, combine the vanilla cake mix, eggs, vegetable oil, buttermilk, and vanilla extract. Blend using a handheld or stand mixer until the batter is thick, creamy, and smooth. Set aside while you prepare the filling.

In a separate bowl, stir together the brown sugar, all-purpose flour, ground cinnamon, and melted butter. Mix until you have a soft, sandy cinnamon mixture that will swirl beautifully through the cake.

Generously coat a 10–12 cup Bundt pan with nonstick spray. Pour half of your prepared cake batter into the pan. Sprinkle half of the cinnamon swirl mixture over the batter and use a toothpick or skewer to gently swirl the cinnamon layer through the cake.
Add the remaining batter, top with the rest of the cinnamon mixture, and swirl again.

Bake for 40–45 minutes or until a toothpick inserted into the center comes out with only a few moist crumbs. Let the cake cool in the pan for at least 15 minutes, then carefully invert it onto a cooling rack or serving plate. Allow it to cool completely before adding the glaze.

Beat the softened cream cheese and butter together until smooth and creamy. Mix in the vanilla extract, then gradually add the powdered sugar until the glaze is thick yet pourable. If needed, add a splash of milk or microwave it for a few seconds to thin.

Pipe or drizzle the cream cheese glaze over the cooled cake using a zigzag motion.
Before the glaze sets, mix the granulated sugar and cinnamon together, then sprinkle it generously over the top for the signature snickerdoodle finish.