Preheat the oven to 350 degrees F. Line a 12-cup muffin tin with paper liners.
PREPARE THE STREUSEL:
In a medium bowl combine the flour, pecans, sugars, and cinnamon. Cut 2 tablespoons butter into small pieces- add to the mixture and use a pastry blender to cut the butter into the mixture until the mixture comes together in crumbles. Measure ¾ cup of the streusel mixture and set it aside to go inside the muffin batter. Melt the remaining 2 tablespoons of butter and stir it into the remaining streusel in the bowl. This will be the topping for the muffins.
MAKE THE MUFFINS:
In a large bowl, combine the flour, sugar and salt. Cut the butter into ½-inch pieces and sprinkle evenly over flour mixture. Using a pastry blender, cut the butter into the flour mixture until the mixture is crumbly. Add the baking powder, baking soda and nutmeg and stir to combine.
In a liquid measuring cup, whisk together the sour cream, egg and vanilla. Add the liquids and pumpkin to the flour mixture and stir just until combined. Add the reserved ¾ cup of the streusel mixture to the flour mixture and stir until the streusel is just distributed throughout batter.
ADD THE STREUSEL:
Divide the batter among 12 muffin cups and sprinkle generously with the streusel topping, crumbling with your hands to make some big chunks. Bake 22 to 24 minutes, or until a toothpick inserted into the center of the muffins comes out clean. Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
ADD THE GLAZE:
In a medium bowl, whisk the powdered sugar, milk and vanilla extract together. Drizzle the glaze over the muffins. Store in a sealed container for up to 2 days.