Preheat your oven to 350°F (180°C). Grease a 9-inch round cake pan with cooking spray.

In a large mixing bowl, combine the flour, granulated sugar, baking powder, baking soda, and salt. Mix well to combine.

In a separate mixing bowl, whisk together the buttermilk, eggs, vegetable oil, and vanilla extract until smooth.

Add the wet ingredients to the dry ingredients and stir until just combined. Be careful not to overmix the batter.

In a small mixing bowl, combine the cinnamon, melted butter, and brown sugar. Stir until well combined.

Pour half of the cake batter into the prepared cake pan. Spread it evenly over the bottom of the pan.

Sprinkle half of the cinnamon mixture over the cake batter.

Pour the remaining cake batter over the cinnamon mixture, spreading it evenly over the top.

Sprinkle the remaining cinnamon mixture over the top of the cake batter.

Bake the cake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Remove the cake from the oven and allow it to cool in the pan for 10 minutes.

Run a knife around the edges of the cake to loosen it from the pan, then invert the cake onto a wire rack to cool completely.