Preheat the oven to 350 degrees. Line a 12-cup muffin-tin with paper liners.
In a medium bowl, combine chopped pecans, brown sugar, and cinnamon. Mix well and set aside.
In a large bowl, combine flour, sugar, and salt. Cut 8 tablespoons of butter into ½ inch pieces and sprinkle evenly over flour mixture. Using a pastry blender, cut the butter into the flour mixture until butter is oat-sized.
Remove 1 cup of the flour-butter mixture and stir into reserved brown sugar mixture until combined to make streusel. Scoop out 3/4 cup of streusel for muffin batter and set aside.
Cut the remaining tablespoon of butter into the remaining streusel until streusel resembles a crumble, then set aside for topping muffins. Add baking powder and baking soda to remaining flour mixture and stir to combine.
In a liquid measuring cup, whisk together sour cream, egg, and vanilla. Add liquids to flour mixture and stir until just combined. Add the 3/4 cup reserved streusel to flour mixture and stir until streusel is just distributed throughout batter. The batter will be thick.
Divide batter among 12 muffin cups and sprinkle generously with streusel topping, pressing lightly with the back of a spoon, so that streusel sinks slightly into batter.
Bake 22 to 24 minutes, or until a toothpick inserted in the center of muffin comes out clean. Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.