Start by preparing the streusel topping. To do this, mix in a medium bowl brown sugar, flour, ground cinnamon, salted butter (cold!) cut into small pieces, and chopped toasted hazelnuts. Mix with your fingers to obtain the moist crumbs that resembled coarse wet sand. Reserve in the fridge.

Next, make the brown butter. Place the unsalted butter in a small saucepan, and heat it on medium heat for about 5 minutes. The melted butter will go through the foaming stage, then it will turn out to be brown with little brown flecks and nutty flavor. Pour brown butter (yes, with solids) into a small bowl to stop cooking. Let it cool to room temperature.

Preheat oven to 300 degrees F/150 degrees C. Butter and flour a 9 x 5-inch loaf pan or 23 x 13 cm baking pan.

To make the bread batter, pour brown butter in a large mixing bowl, add sugar, and beat with an electric mixer for 1 to 2 minutes. Next, add eggs one at a time, mixing after each addition. Combine all-purpose flour, baking powder, spices, and salt in a separate bowl and sift dry ingredients. Add flour mixture to the principal preparation and gently mix with a rubber spatula. Grate the unpeeled apples (no need to wring them out) and add them to the batter. Mix just to combine. Add maple syrup and gently mix. Pour batter into the prepared pan. Take the crumble out of the refrigerator and sprinkle the streusel on top of the batter. Lightly press the crumb topping to adhere to the batter.

Bake the bread for 80 to 90 minutes. Then, insert a toothpick in the center of the loaf: if it comes out clean, the bread is ready. Let the sweet bread rest in the pan for 30 minutes. Then remove it and let it cool on a wire rack. Dust the cooled loaf with a little bit of icing (powdered) sugar if desired.