Preheat the oven to 350°F. Spray the bottom of an 8-inch square pan with nonstick cooking spray.
In a small bowl, combine all of the streusel ingredients with a fork until crumbly. Set aside.
In a large bowl, stir together the flour, baking powder, baking soda, salt, brown sugar, and walnuts. In a medium bowl, stir together the maple syrup, oil, milk, eggs, and maple extract. Stir in the sour cream. 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.
In a small bowl, combine the sugar and maple syrup until a thick, pourable icing forms. Drizzle over the warm cake. Cut into squares and serve. Store leftovers inside an airtight container at room temperature for up to 4 days.
