Throw your old muffin recipe away! This one is the best! The topping is optional, but it is the way we like it. Adding blueberries to the top before baking make these muffins look like they are bursting with berries.
Preheat oven to 375 degrees. Line muffins tins with paper liners and spray with cooking spray.
To make the topping, combine flour, brown sugar and butter together in a bowl and using a pastry cutter or 2 knives, cut butter into flour and sugar until a course meal texture. Add nuts and set aside.
For the batter, combine dry cake mix, flour and baking powder in large bowl. Mix milk, eggs and oil together; add to dry mixture and stir until dry ingredients are moistened. There will be lumps.
Separate 1/2 cup of blueberries and set aside. Fold remaining blueberries into batter gently.
Spoon batter into pregared muffin cups, filling 1/3 full. Place a few blueberries that were reserved on top of each muffin and 1 tablespoon or more of crumble topping.