Make the crumb topping. In a medium bowl, whisk together the flour, light brown sugar, cinnamon, and salt. Pour in the melted butter and vanilla and stir with a fork until large, moist clumps form. Continue breaking them up to form smaller pea sized clumps. Set aside.
In a large bowl, whisk together the pumpkin purée, granulated sugar, dark brown sugar, oil, eggs, vanilla, and sour cream until smooth. In a small bowl add the dry ingredients, flour, baking powder, baking soda, pumpkin pie spice, and salt. Mix with a fork to combine then add them to the wet ingredients and fold with a non-stick spatula until just combined, with no dry streaks remaining.
Using a 2.5-3 tablespoon cookie scoop, scoop the batter onto the prepared baking sheet, leaving at least 2 inches between each one. Sprinkle the crumb topping generously over each scoop, pressing gently so it sticks.
Bake for 10 to 14 minutes, until the tops spring back a toothpick inserted in the center comes out clean. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Sprinkle with powdered sugar.
Let the baking sheet cool, line with fresh parchment if needed, and repeat with the remaining batter and crumb topping.
