Preheat oven to 350 degrees F. Lightly spray an 8" square baking pan with non-stick spray and set aside.
Topping:
To prepare the topping, start by melting the butter in a small pot over medium-low heat. Add the brown sugar, vanilla extract and salt. Stir until smooth. Remove from heat and stir in flour, then let cool completely.
Batter:
Combine the flour, brown sugar, baking powder, baking soda, pumpkin pie spice and salt in a large bowl, then whisk to combine.
Combine the pumpkin puree, eggs, vegetable oil, sour cream (OR plain greek yogurt) and vanilla in another large bowl and whisk to combine.
Whisk egg mixture into flour mixture just until smooth, and pour into prepared pan. Crumble the topping and sprinkle it over the batter.
Bake for 40 minutes or until a wooden toothpick inserted in the center comes out clean. Let cool completely in pan on a wire rack.
For the icing:
Whisk together the powdered sugar and milk until smooth and then drizzle over the cooled cake. Enjoy!