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.
Scoop the batter into the prepared loaf pan. Top with the walnut crumble. Bake for 55-65 minutes total.Cool in the pan for 20 minutes. While the cake cools make the maple glaze.
Make the maple glaze: Sift powdered sugar directly into the bowl with the warm brown butter. Add maple syrup by the tablespoon (15 ml), as needed to create the glaze consistency you want.