Combine Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Mix Butter and Sugars: In large bowl, throughly mix melted butter with the brown and granulated sugar until well combined.
Add Eggs, Vanilla, Sour Cream and Milk: Add the eggs and vanilla to the to butter mixture and mix well. Stir the sour cream and milk into the batter until smooth
Add Flour Mixture: Stir the dry ingredients into the batter, about one cup at a time, mixing until just absorbed. The batter will be thick.
Filling: In a small bowl, combine the cinnamon and brown sugar. Stir in pecans.
Assemble: Pour about half of the batter into the prepared pan and spread it evenly over the bottom. Sprinkle the filling evenly over the batter. Carefully spread the remaining batter over the top.
Add Crumble: Evenly sprinkle the crumble topping over the batter, gently pressing it in so it adheres.
Bake: Bake for 50–60 minutes, or until a cake tester inserted into the center comes out clean. Let cool for about 5 minutes (if you can wait!) before serving.