Preheat oven to 350°F | 176°C. Grease a 9" square pan, or spray with cooking spray.
Chop walnuts. I usually chop nuts for recipes by hand, but for this recipe, you want them a little finer. I use a small food processor for this step, processing the nuts a handful at a time, pulsing quickly a few times with each batch. You want most of the walnuts to be finely chopped, but you don't want to make nut butter. It's good to have a few bigger pieces here and there. It gives the Crumb Topping a more interesting texture, too.
Whisk together nuts, brown sugar, 2 tablespoons of flour, cinnamon, and salt in a medium bowl.
Add vanilla and melted butter and mix until thoroughly combined. Set aside.
In a large mixing bowl, use a wooden spoon to break down butter. It is critical that the butter be at least 65°F [18.3°C] or it will be too hard to mix the cake. Optimally, butter should be about 70°-72°F.
In a large mixing bowl, use a spatula to combine sugar and butter. It doesn't need to be pretty; just mix it in thoroughly.
Add eggs to butter and sugar mixture one at a time, stirring thoroughly to combine after each addition.
Stir in vanilla and sourdough starter. Set aside.
In a medium bowl, whisk or sift together 2 cups of flour, baking powder, baking soda, and salt.
Add dry ingredients to Starter mixture and stir to combine. Add milk and mix until smooth.
Stir in orange zest and cranberries.
[A] Spread one-half cake batter in the bottom of a prepared pan. [B] Sprinkle one-half crumble mixture over the top. Press the crumble down very lightly with your hands. [C] CAREFULLY distribute the rest of the batter on top of the first crumb layer. The idea is to mix in as little of the crumb mixture into the cake batter as possible. I do this by dropping dollops of batter over the top of the crumb layer, and then carefully spreading it to the edges using a frosting spatula or the back of a spoon. [D] Sprinkle the second half of the crumble mixture over the top. Press the crumble down very lightly with your hands.
Bake in preheated oven for 35 minutes. Open the oven door and gently cover the cake top with a piece of foil. Basically, just lay the foil over the top. This will keep the topping from over browning during the final minutes of baking. Close the oven and continue baking for an additional 10-20 minutes (for a total of 45-55 minutes cooking time), or until a skewer comes out clean.
Remove from oven and cool on rack.
This coffee cake can be stored at room temperature for several days. It also freezes well.
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