Set the oven to 350°F (175°C) to preheat.

In a large bowl, whisk together the vanilla cake mix, eggs, vegetable oil, buttermilk, and vanilla extract. Use a hand or stand mixer to beat until the batter is thick and completely uniform. Leave aside.

In a small bowl, mix the brown sugar, all-purpose flour, cinnamon, and melted butter until the mixture is uniform.

Coat a Bundt pan generously with nonstick cooking spray. Add half of the cake batter to the pan. Drop half of the cinnamon swirl mixture by spoonfuls onto the batter. Drag a toothpick or skewer through the batter to create swirls. Add the rest of the cake batter on top, then the remaining cinnamon swirl mixture, and swirl once more.

Bake for 40–45 minutes, until a toothpick inserted in the center comes out with just a few crumbs and no wet batter. Take out of the oven and allow the cake to cool in the pan for at least 15 minutes.

After 15 minutes, turn the cake out onto a serving plate and let it cool fully.

In a bowl, use a hand mixer to beat the softened cream cheese and softened butter until smooth. Add the vanilla extract and powdered sugar, and keep mixing until the frosting reaches a spreadable consistency. If it is too thick, stir in a tablespoon of milk or microwave for 20 seconds.

Put the cream cheese glaze into a piping bag or zipper bag, and snip a small triangle from the corner. Pipe in a zigzag pattern over the cake.

While the glaze is still wet, combine the cinnamon and sugar in a small bowl and stir until evenly mixed. Sprinkle as much as you like over the cake. Serve, or keep in the refrigerator for up to 4 days.