01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
02 - Combine brown sugar, flour, and cinnamon in a small bowl. Cut in cold butter using a 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 until well blended.
04 - Whisk eggs, oil, milk, and vanilla extract in a separate bowl until fully incorporated.
05 - Pour wet ingredients into dry ingredients. Fold gently with a spatula just until combined—do not overmix or muffins will be tough.
06 - Divide batter evenly among muffin cups, filling each about three-quarters full.
07 - Sprinkle the crunch topping generously over the surface of each muffin, pressing lightly to adhere.
08 - Bake for 20 to 22 minutes until golden and a toothpick inserted into the center comes out clean.
09 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
10 - Dust with powdered sugar just before serving if desired.