Preheat oven to 350°F (175°C). Grease the baking pan.
Mix sweet potatoes, sugar, butter, eggs, and vanilla in a large bowl.
In a separate bowl, combine flour, baking powder, and salt.
Gradually add dry ingredients to the sweet potato mixture, alternating with milk.
Pour half the batter into the pan. Sprinkle with cinnamon-sugar mix.
Add remaining batter and swirl with a knife for a marbled effect.
Bake for 35-40 minutes until a toothpick comes out clean.
Cool before frosting with cream cheese icing.