1Preheat oven to 375 degrees F (190 degrees C). Spray a 9x13-inch baking dish with cooking spray. Sprinkle cinnamon sugar into the bottom of prepared baking dish.
2Combine brown sugar, walnuts, softened butter, 1/4 cup flour, and cinnamon in a bowl using a fork until streusel is crumbly.
3Mix 3 cups flour, baking powder, and salt together in a separate bowl. Beat eggs in a third bowl using an electric mixer until frothy; add white sugar and melted butter and beat until creamy. Add milk, sour cream, and vanilla extract and beat until smooth. Stir flour mixture into milk-butter mixture until batter is well blended.
4Spread 1/2 of the batter into the prepared baking dish; top with 1/2 of the streusel mix. Spread remaining batter over streusel layer and top with remaining streusel.
5Place dish on the center rack in the preheated oven; bake until a fork inserted in the center comes out clean, 15 to 20 minutes.