Make the crumble topping. Work in the cubes of butter until the mixture comes together into coarse pieces of sand, with some larger pieces mixed in. Set the bowl in the fridge to chill while preparing the apple cake.
Beat the softened brown butter and sugar until well combined. On low, beat in the eggs, vanilla and maple extracts. Mix in dry ingredients to the wet alternating with the buttermilk. Fold in chopped apples.