Grease an 8-9 inch springform pan* and line the bottom with parchment paper round, or grease really well. Preheat oven to 350°.
Make your cinnamon crumb topping by whisking together all the dry ingredients, then add the melted butter and vanilla, stirring with a fork or spoon until mixture is evenly moist and presses together when squeezed in hand. Place in fridge while making the cake.
In a mixing bowl, cream together butter with sugar until light and creamy, add egg, beating well for about 1 minute. Add vanilla and sour cream, beat until incorporated.
In a small bowl, stir together flour, baking powder, baking soda, and salt. Add to the creamed butter mixture, just until combined.
Spread half the batter on the bottom of the pan, it will be thin. Spread the apples evenly over the batter, sprinkle with cinnamon. Sprinkle one cup cinnamon crumb mixture over the apples. Spread remaining batter over the crumbs, it doesn't have to be perfect! Spread remaining crumb mixture evenly over the top.
Bake on middle rack of oven, for 35-40 minutes, checking at 35 minutes using a cake tester or toothpick. If it comes out clean it's done. Remove from oven, cooling 15-20 minutes on wire rack.
Before removing the springform sleeve, run a knife around the edges of the cake.
If desired, whisk together powdered sugar and maple syrup (or apple cider) and drizzle over cake.
If you do not have a springform pan, you can certainly make this cake in a 8-9" square pan or round cake pan. Apple Crumb Cake keeps best in fridge in airtight container for 3-5 days, may be frozen up to 3 months. Adapted from OMG Chocolate Desserts