Preheat the oven to 350 degrees. Grease a 9” x 9” square or circle cake tin or baking dish.
In a medium sized mixing bowl, combine together the flour, cinnamon, nutmeg, kosher salt, baking powder, and baking soda.
Using a stand mixer with paddle attachment, cream together the butter, dark brown sugar, and maple syrup.
Add one egg at a time, beating well after each addition. Add the vanilla, sour cream, and milk and mix of medium speed for one minute until well combined.
Working in batches, gradually add the flour mixture, beating well after each addition and scraping down the sides. Set aside to make the filling and topping.
To create the filling, in a small bowl, combine together the flour, oats, brown sugar, maple sugar, cinnamon, walnuts, and pecans. The oats and butter will be mixed in later to the topping.
Pour in half of the batter to the prepared pan and sprinkle 1/4 of the filling to evenly cover the batter.
Pour the remaining batter over the filling.
To make the topping, Add the oats and melted butter to the remaining filling, combining well until it all is evenly coated with butter. Sprinkle the topping evenly over the cake.
Bake for 50-60 minutes or until a knife comes out clean when inserted in the center.