Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a large bowl, mix the flour, sugar, baking powder, and salt until combined.
In another bowl, whisk together melted butter, milk, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
In a small bowl, combine brown sugar and cinnamon for the filling.
Pour half of the cake batter into the prepared pan, spreading evenly.
Sprinkle half of the cinnamon-sugar mixture over the batter.
Pour the remaining batter on top and swirl in the rest of the cinnamon-sugar mixture.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool for 10 minutes before transferring it to a wire rack.