1. Preheat oven to 350ºF. Grease a 12-cup Bundt pan. Make streusel: In a bowl, combine walnuts, both sugars, cinnamon and salt; set aside.
2. Make cake: In a bowl, whisk together both flours, baking powder and salt. Using an electric mixer on medium speed, beat both sugars and butter until light and fluffy, about 3 minutes. Beat in eggs one at a time, mixing after each addition, then beat in egg yolk. Beat in half of flour mixture, until just combined. Stir in sour cream and vanilla, then fold in remaining flour mixture. Do not overmix. Batter will be thick.
3. Spread two-thirds of batter evenly in Bundt pan. Sprinkle streusel over, then top with remaining batter, gently spreading batter until streusel is entirely covered. Bake until a tester inserted in center of cake comes out clean, 35 to 40 minutes. Let cake cool on a wire rack for 10 minutes, then turn out of pan onto wire rack to cool.
4. Make glaze: Sift confectioners' sugar into a small bowl, add milk and vanilla and mix with a fork until smooth. Drizzle glaze over top of cake, allowing some to drip down sides.
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