Preheat oven to 350 degrees. Spray an 8-inch square or 9-inch round pan with cooking spray.

Make the streusel: Combine the brown sugar, flour, finely chopped walnuts, cinnamon, and melted butter with a fork in a small bowl. The mixture will be crumbly.

Make the cake batter: Stir together the flour, baking powder, baking soda, salt, brown sugar, and walnuts in a large mixing bowl and set aside.

Using a spoon, make a well in the middle of the dry ingredients. Slowly pour in the wet ingredients. Gently stir until all ingredients are combined.

Spoon the first half of the cake batter into the prepared pan. Sprinkle half of the streusel over the cake batter.

Slowly spoon the rest of the batter on top of the streusel. Spread the batter throughout the baking pan. Sprinkle the remaining streusel on top.

Bake at 350 degrees for 30-35 minutes (or until the cake begins to pull away from the sides of the pan, and an inserted toothpick comes out clean.)

Make the maple glaze: Combine the sugar and maple syrup in a small bowl. Blend the sugar and maple syrup together. The glaze should be thin enough to pour over the top of the cake. If it's too thick, add a little warm water, one teaspoon at a time, until you have a pourable glaze.

Once the cake is baked, let it cool in the pan for 10-15 minutes. Drizzle the glaze over the warm cake.

Cut into squares or wedges and serve.