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