Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition, followed by the vanilla extract.

In another bowl, whisk together the flour, baking powder, and ground cinnamon.

Gradually add the dry ingredients to the wet mixture alternately with the milk until everything is well combined.

Pour half of the batter into the greased cake pan and sprinkle the brown sugar cinnamon mixture over the top.

Add the remaining batter on top and swirl it gently with a knife to create the cinnamon swirl effect.

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

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.