Preheat oven to 350 degrees F.
Spray doughnut pans with nonstick cooking spray. Set aside.
Add unsalted butter, granulated sugar, light brown sugar to stand mixer. Beat on medium until smooth and creamy, about 2 minutes.
Add eggs one at a time and beat on low to incorporate after each addition. Add vanilla beat extract and beat on low until incorporated.
Add the all-purpose flour, baking powder, baking soda, and fine sea salt to a medium bowl. Whisk to combine.
Add 1/2 flour mixture to stand mixer. Beat on low just until incorporated. Add sour cream. Beat on low just until incorporated. Add remaining flour mixture. Beat on low just until incorporated. Do not over mix.
Combine light brown sugar and ground cinnamon in small bowl. Set aside.
Add doughnut batter to large gallon size plastic storage bag and cut a small hole in corner of bag. Seal the bag and squeeze the batter through the opening into the doughnut pan cavities about 2/3 of the way to the top. Smooth batter with wet finger. Sprinkle doughnuts with reserved cinnamon sugar. Swirl with tip of knife to incorporate streaks of cinnamon sugar in doughnuts.
Bake doughnuts in hot oven for 5 minutes. Remove from oven and carefully and quickly sprinkle walnut streusel over doughnuts. Press lightly. Return to oven and continue baking until doughnuts are firm to the touch.
NOTE: baking times will vary with doughnut pans. Small pans will require 10 to 12 minutes baking. Large pans will require 18 to 20 minutes.
Remove doughnut pans from oven and cool on wire rack for 15 minutes. Remove doughnuts from pans to cool completely.