Preheat oven to 350ºF (180ºC).
Grease or spray well a 7x11-inch cake pan (or 9-inch square or round pan) and line the bottom with parchment paper.
Beat ½ cup unsalted butter in a large bowl and gradually add 1 cup light brown sugar. Beat for 3 minutes, until creamy.
Add 2 eggs, one at a time, beating well after each addition.
Add ⅔ cup pumpkin puree, 2 teaspoons ground cinnamon, 1 teaspoon ground ginger¼ teaspoon ground allspice (OR use 3 teaspoons pumpkin pie spice mix instead of above spices) and ½ teaspoon vanilla extract and mix just until combined.
Sift 1 ½ cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda and ½ teaspoon salt. You can do it directly or sift them in a different bowl before adding. Add the dry ingredients in 2 parts, alternating with ⅓ cup sour cream in 1 part. This means that you add half the dry ingredients, the total amount of sour cream, and the rest of the dry ingredients.
Mix lightly with the beater each time to incorporate, and at the end with a spatula only until combined and no dry spots remain, especially in the bottom. Don't overbeat at this point to ensure a light crumb.
Pour the batter into the prepared pan, and smooth the top.
Sprinkle the cold streusel over it.
Bake for about 30 minutes, until a tester comes out dry. The top will be dark because of the cinnamon and brown sugar, but it's not burning. You can cover the cake for the last 5-10 minutes with a piece of foil to prevent it from turning very dark.
Let cool in the pan on a wire rack for 15 minutes, and carefully run a smooth blade knife around the edges.
Lift the paper carefully and transfer the cake to the wire rack to cool completely then peel off the paper very carefully.
Drizzle with the icing, let dry out a little, cut into squares and enjoy!