• Preheat the oven to 350°F. Spray the bottom of a metal 8-inch square pan with nonstick cooking spray.

Make the streusel:

• In a small bowl, combine all of the streusel ingredients with a fork until crumbly.

Make the cake:

• In a large bowl, stir together the flour, baking powder, baking soda, salt, cinnamon, ginger, nutmeg, brown sugar, and granulated sugar. In a small bowl, stir together the pumpkin puree, oil, sour cream, and eggs. 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. Glass and ceramic pans will take longer to bake than metal. Cool for 15 minutes.

Make the topping:

• 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, covered, at room temperature for up to 4 days.