In a large bowl, combine flour, sugar, baking powder, cinnamon and salt.
2 cups flour, ½ cup sugar, 1 Tablespoon baking powder, 1 teaspoon cinnamon, ¼ teaspoon salt
In a different bowl, combine egg, sour cream, oil, milk and vanilla.
1 egg, 1 cup sour cream, ½ cup oil, ½ cup milk, 1 teaspoon vanilla
Add egg mixture to flour mixture. Stir just until combined.
Gently fold in blackberries.
2 cups blackberries
Place mixture into muffin tins lined with paper or sprayed lightly with a nonstick spray.
Bake in a preheated 400° oven for 18-20 minutes.
Let cool slightly on a baking rack. Remove from muffin tin and continue cooling.
Notes
Use fresh baking powder - baking powder can lose its effectiveness and if old, your muffins might not rise properly.
Fully preheat your oven - I use a separate oven thermometer in my oven to make certain my oven has reached the proper temperature. My preheat light often turns off before the oven has reached full temperature so I always give it extra time.
Do not overmix batter. Just stir until everything is mixed in. Over-stirring can cause tough muffins.
Use an ice cream scoop for evenly sized muffins. Plus it makes putting the batter in the tins easy peasy!