Streusel: 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.
Coffee Cake: Heat oven to 350 degrees 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 wooden pick inserted in the center comes
out clean. Cool for 15 minutes.
Topping: In a small bowl combine the powdered sugar and maple syrup
until a thick, pourable icing forms. Drizzle icing over the warm cake. Cut
into squares and serve.
Store leftovers, covered, at room temperature for up to 4 days.