Preheat the oven to 325 degrees. Butter a 13 x 9 inch baking pan and line with parchment paper.
In a small bowl for the streusel, mix together brown sugar, pecans and cinnamon. Set aside.
In another bowl, whisk together the flour, baking soda, baking powder, spice and cinnamon.
Using a stand or hand mixer, beat the soft butter with the sugars until light and fluffy. Scrape down periodically.
Add the eggs one at a time and beat again until well combined.
Add the vanilla extract and pumpkin puree and mix for about 2-3 minutes.
Add half of the flour mixture and half of the sour cream. Mix well before adding the remaining flour and sour cream.
Mix until combined and no flour is visible.
Transfer the batter to the prepared pan and spread out evenly with a spatula.
Sprinkle some of the streusel topping over the batter and place in the oven
Bake at 325 degrees for about 30-35 minutes. Check for doneness by gently pushing two fingers on top of the cake. It should spring back.
Let cool before frosting.
Mix the powdered sugar and milk together until smooth and runny.
Drizzle the frosting over the coffee cake and then slice into squares.