For the topping

1. Whisk flour, brown sugar and cinnamon in a medium size bowl. Add the cubed butter. With a pastry cutter, fork, knife or hands cut the butter into the mixture until it resembles medium size crumbs. Set aside. 
2. Chop the apples. Toss with cinnamon and brown sugar and set aside.

For the cake

1. Preheat oven to 350 F. Lightly grease an 8 inch springform pan. Set aside.
2. Whisk flour, cinnamon, baking powder and baking soda in a medium size bowl. Then set aside.
3. Into a mixer bowl add the eggs, oil and sugars. Beat until combined.
4. Add the sour cream and milk. Beat until combined.
5. Add the flour mixture to the mixer and beat until flour is just incorporated. The batter will be thick! 
6. Pour the cake batter into the prepared cake pan.
7. Add the chopped apples and lightly press the apples into the batter.
8. Sprinkle with the oatmeal topping.
9. Bake 40-50 minutes or until toothpick inserted comes out clean and topping is dark golden brown.

For the glaze

1. To a small bowl add melted butter, maple syrup, powdered sugar and milk. Whisk until combined and smooth. Add more milk or powdered sugar until desired consistency. Then drizzle over cake.