Preheat oven to 350°F (175°C) and grease an 8-inch round cake pan.
In a mixing bowl, whisk together flour, sugar, baking powder, cinnamon, and salt.
In a separate bowl, whisk together melted butter, milk, egg, and vanilla extract.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Pour the batter into the prepared cake pan and smooth the top.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Remove from oven and sprinkle cinnamon-sugar mixture on top while warm.
Let cool before slicing and serving.