Prepare the Cake:
Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch baking dish or line it with parchment paper.
In a large mixing bowl, combine the vanilla cake mix, eggs, vegetable oil, buttermilk, and vanilla extract. Use a hand mixer or stand mixer to beat the mixture until smooth, about 2 minutes.

Prepare the Cinnamon Swirl:
In a small bowl, combine the brown sugar, flour, cinnamon, and melted butter. Mix until the ingredients form a crumbly mixture.

Layer the Cake:
Pour half of the cake batter into the prepared baking dish. Spread it evenly. Then sprinkle half of the cinnamon swirl mixture evenly over the batter. Top with the remaining cake batter and swirl the cinnamon mixture into the cake using a knife or skewer, creating a marble effect.

Bake the Cake:
Bake the cake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for about 10-15 minutes before transferring it to a wire rack to cool completely.

Make the Cream Cheese Glaze:
While the cake is cooling, make the cream cheese glaze. In a medium mixing bowl, beat the softened cream cheese, butter, and vanilla extract until smooth. Gradually add the powdered sugar and continue to beat until the glaze is thick and smooth.

Make the Cinnamon Sugar Mixture:
In a small bowl, combine the granulated sugar and cinnamon for the topping.

Assemble the Cake:
Once the cake has cooled completely, spread the cream cheese glaze over the top. Sprinkle the cinnamon sugar mixture evenly on top of the glaze for a sweet finishing touch.

Serve and Enjoy:
Slice the cake into squares and serve. Enjoy the cinnamon-sweet goodness of Snickerdoodle Cake!