01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
02 - In a small bowl, combine brown sugar, flour, and cinnamon. Cut in cold butter using a fork or pastry blender until mixture resembles coarse crumbs. Stir in nuts if using. Set aside.
03 - In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and sugar until well blended.
04 - In another bowl, whisk eggs, oil, milk, and vanilla extract until fully combined.
05 - Add wet ingredients to dry ingredients. Mix gently with a spatula until just combined. Do not overmix.
06 - Divide batter evenly among muffin cups, filling each about 3/4 full.
07 - Sprinkle the crunch topping generously over each muffin.
08 - Bake for 20-22 minutes, or until a toothpick inserted into the center comes out clean.
09 - Cool muffins in pan for 5 minutes, then transfer to wire rack to cool completely. Dust with powdered sugar before serving if desired.