MAKE STREUSEL Process flour, butter, brown sugar, cinnamon, and pecans in food processor until finely ground. Transfer streusel to bowl and wipe out food processor.
MAKE BATTER Adjust oven rack to middle position and heat oven to 325 degrees. Grease and flour 12-cup nonstick Bundt pan. Combine flour, baking powder, baking soda, and salt in bowl. In food processor, blend sugar, eggs, sour cream, and vanilla until smooth, about 1 minute. With machine running, slowly pour in butter until incorporated, then add flour mixture and pulse until just combined.
LAYER AND BAKE Pour half of batter into prepared pan and top with streusel mixture. Cover with remaining batter, using rubber spatula to smooth surface. Bake until golden brown and toothpick inserted in center comes out clean, 50 to 60 minutes. Cool cake in pan 20 minutes.
GLAZE While cake is cooling, whisk confectioners’ sugar, milk, and vanilla in bowl until smooth. Turn out cake onto rack set inside rimmed baking sheet. Pour glaze over warm cake. Cool completely, at least 2 hours. Serve. (Cake can be stored at room temperature, covered in plastic wrap, for 2 days.).