In a small bowl combine brown sugar, flour, walnuts and ground cinnamon, and whisk together using a fork. Use the fork to stir while you pour in the melted butter until a crumb-like texture forms. Break up any large crumbs with the fork.
Preheat oven to 350°F. Spray the bottom of an 8-inch square pan with nonstick cooking spray.
In a large bowl, stir together the flour, baking powder, baking soda, salt, brown sugar and walnuts.
In a small bowl, stir together the maple syrup, vegetable oil, milk, eggs and maple extract, if desired. Stir the sour cream into the wet ingredients until combined.
Make a well in the center of the dry ingredients, then pour in the wet ingredients. Gently stir until just combined.
Spoon half of the batter into the prepared pan. Sprinkle with half of the streusel. Spread the remaining batter over the streusel. Sprinkle with the remaining streusel.
Bake for 35 minutes, or until a toothpick inserted in the center comes out clean. Cool for 15 minutes.