Instructions

Adjust oven rack to middle position and heat oven to 350 degrees. Grease 12-cup muffin tin and set aside.
Process nuts, brown sugar, and cinnamon in food processor until nuts are size of sesame seeds, about ten 1-second pulses. Transfer mixture to medium bowl.
Return bowl and metal blade to food processor, add flour, granulated sugar, and salt and process until combined, about five 1-second pulses. Sprinkle butter evenly over flour mixture and process until butter is oat-sized, about eight 1-second pulses. Remove 1 cup of flour-butter mixture and stir with fork into reserved brown sugar mixture until combined to make streusel. Set aside 3/4 cup of streusel for muffin batter and remaining portion for topping muffins.
Add baking powder and baking soda to remaining flour mixture in food processor bowl and process until combined, about five 1-second pulses. Whisk together sour cream, egg, and vanilla; add to flour mixture. Process until batter is just moistened, about five 1-second pulses. Add 3/4 cup reserved streusel to flour mixture and process until streusel is just distributed throughout batter, about five 1-second pulses.
Divide batter among 12 muffin cups and sprinkle with streusel, pressing lightly so that streusel sinks slightly into batter. Bake until toothpick inserted in center of muffin comes out with several crumbs clinging to it, about 18 minutes, rotating pan from front to back halfway through baking time. Place a sheet of parchment paper beneath a wire rack. Cool muffin tin on wire rack for 2 minutes. Using tip of paring knife, loosen muffins and gently transfer from tin to wire rack.
Whisk confectioners' sugar and water in medium bowl until smooth. Spoon 2 teaspoons glaze over each muffin, letting glaze run down sides of muffins.