Preheat the oven to 350℉. Lightly spray a 9 inch round cake pan with cooking spray and set aside.
In a large bowl, cream together the butter and brown sugar until light and fluffy. Mix in the egg and vanilla.
In a medium bowl, mix together the flour, baking powder and cinnamon.
Alternate between mixing the flour mixture and milk into the butter mixture. Mix until just combined.
Pour the cake batter into the prepared baking pan. Arrange the apple slices over top in a spiral formation.
In a small bowl, mix together the cinnamon sugar ingredients. Sprinkle over the top of the cake.
Bake the cake in the preheated oven for around 50 minutes, or until a toothpick inserted into the center comes out clean.