Step 1

Preheat your oven to 350 degrees F. Grease a 9x9 inch cake pan with nonstick spray or line with parchment paper.

Step 2

For the streusel - using a pastry cutter or fork, combine the cold butter, flour, brown sugar, and cinnamon. You should end up with a bowl of what looks like wet sand, with lots of big chunks. Don't try to smooth it out. Put in fridge to chill.

Step 3

For the walnut sugar filling - combine the walnuts, brown sugar, white sugar and cinnamon. Set aside.

Step 4

For the cake - In a large bowl or stand mixer, cream the butter and sugar fitted with the paddle attachment for 2 to 3 minutes, until light. Add the eggs 1 at a time, then add the vanilla.

Step 5

In a separate bowl, sift together the flour, baking powder, and salt. With the mixer on low, add half the flour mixture to the batter until just combined. Then add in the sour cream. Finish by adding the remaining flour mixture and stir with a spatula to be sure the batter is completely mixed.

Step 6

Spoon half the batter into the pan and spread it out with a spatula or knife. Sprinkle half the walnut sugar filling over the batter, and approximately 1/3 of the streusel topping. Spoon the rest of the batter in the pan, spread it out, and scatter the remaining streusel on top. Bake for 55 to 60 minutes, until a cake tester comes out clean.

Step 7

Let cool on a wire rack for at least 15 minutes. Carefully transfer the cake, streusel side up, onto a serving plate. Sprinkle with icing sugar and devour!