1/2 cup pecans
1/4 cup packed dark brown sugar
1 teaspoon ground cinnamon
2 cups unbleached all-purpose flour
1 cup granulated sugar
1 teaspoon salt
1 stick unsalted butter, cut into 1/2-inch pieces and softened (+ 1 tbsp)
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
3/4 cup sour cream
1 large egg
1 teaspoon vanilla extract
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 a medium bowl.
Return bowl and metal blade to food processor, add flour, granulated sugar, and salt and process until combined. Sprinkle the 1 stick of 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 pecan/brown sugar mixture until combined to make streusel.
Set aside 3/4 cup of the streusel for muffin batter and save the remaining portion for topping muffins.
Note: At this point I added an additional tablespoon of butter to the streusel for topping the muffins.
Transfer remaining flour mixture, and 3/4 cup streusel to a large bowl. Mix until combined, then make a well.
In a separate bowl, whisk together sour cream, egg, and vanilla. Add to flour mixture, and gently stir until combined.
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 22 minutes.
Cool muffin tin on wire rack for 2 minutes. Using tip of paring knife, loosen muffins and gently transfer from tin to wire rack; cool for 5 minutes and serve warm.