Preheat oven to 350°F. In the bowl of a food processor, combine 1 cup pecans, brown sugar and ½ cup flour then process until well blended. Place pecan mixture in a large mixing bowl then make a well in the center of the mixture. In a separate bowl, beat eggs until foamy then stir in ½ cup melted butter and vanilla. Add egg mixture to the well in the dry ingredients then mix until well combined. Set batter aside. In a large bowl, combine all ingredients for streusel, stirring together with a fork to mix well. Generously grease a muffin pan and spoon batter into the cups, filling each cup ? full. Sprinkle pecan streusel on top of each muffin, pressing lightly on each batch of streusel to make sure it sticks to the batter below. Bake 20–25 minutes or until a toothpick inserted in the center of a muffin comes out clean. Remove from oven, take muffins out of pan and place on cooling racks. Serve warm.