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

In a mixing bowl, whisk together the flour, baking powder, cinnamon, and salt.

In another bowl, cream the softened butter and sugar until light and fluffy.

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

Gradually add the dry ingredients to the wet mixture, alternating with the milk, and mix until just combined.

Pour the batter into the prepared cake pan and smooth the top.

In a small bowl, mix together the granulated sugar and cinnamon for the topping. Sprinkle it evenly over the batter.

Bake in the preheated oven for 25-30 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.