In a large mixing bowl, mix the butter, sugar, egg, vanilla extract, sour cream and milk.
In a medium mixing bowl, mix the flour, baking powder and salt. Slowly stir this into the larger mixing bowl until the batter forms.
Line a casserole slow cooker or a 5 qt or larger slow cooker with parchment paper. Spray cooking oil on the parchment paper.
Pour the batter into the slow cooker.
In a medium mixing bowl, mix the ingredients for the apple cinnamon mixture. Slowly spoon this apple mixture over the top of the batter as evenly as possible.
Next in a smaller bowl, mix the ingredients for the cinnamon topping. Spread this mixture on top of the apples as evenly as possible.
Cover and cook on HIGH for 2 hours or until the sides are golden and the middle has set.
Remove the parchment papers by pulling up on both sides. Allow the cake to cool. Slice and serve.