For the streusel:

1. Mix dry ingredients in a medium bowl.
2. Add butter and work with your fingers or pastry cutter until you have a crumbly, clumpy mixture. Don’t let the butter start to melt.
3. Refrigerate while preparing the cake batter.

For the cake:

1. Preheat oven to 350ºF / 180ºC.
2. Grease or spray well a 7x11-inch cake pan (or 9-inch square or round pan) and line the bottom with parchment paper.
3. Beat butter in a large bowl and gradually add sugar. Beat for 3 minutes, until creamy.
4. Add eggs, one at a time, beating well after each addition.
5. Add pumpkin, spices, and vanilla and mix just until combined.
6. Sift flour, baking powder, baking soda, and salt over this batter (you can do it directly or sift them in a different bowl before adding) in 2 parts, alternating with the sour cream. This means that you add half the dry ingredients, the total amount of sour cream, and the rest of the dry ingredients.
7. 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. 
8. Pour the batter into the prepared pan, and smooth the top.
9. Add the cold streusel over it.
10. 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.
11. Let cool in the pan on a wire rack for 15 minutes, and carefully run a smooth blade knife around the edges. 
12. Lift the paper carefully and transfer the cake to the wire rack to cool completely then peel off the paper very carefully.
13. Drizzle with the icing, let dry out a little, cut into squares and enjoy!

For the icing:

1. Mix both ingredients in a bowl until completely smooth.
2. Adjust liquid to get the desired consistency (more for a more runny icing, less for a thicker).