Preheat oven to 350 degrees F.   Grease and flour 9 x 9 cake pan.

In a small glass, mix cinnamon and brown sugar together. Set aside.

Peal and chop apple. Set aside.

Using an electric mixer, cream the butter and sugar together until light and creamy. About 5 minutes.

Beat in eggs, one at a time, until thoroughly incorporated. Add vanilla extract and milk. Mix well. 

Combine flour and baking powder in another bowl, then add to the liquid ingredients. Mix until smooth. Stir in the chopped apples.

Pour half the batter into the prepared pan. Sprinkle half of the brown sugar/cinnamon mixture on top, and swirl with a knife. Add the remaining batter, and swirl the remaining sugar/cinnamon mixture.

Bake for 30 minutes or until toothpick inserted in the center of the cake comes out clean.