Preheat the Oven: Start by preheating your oven to 350°F (175°C). Line a muffin tin with paper liners or grease it lightly.
Prepare the Dry Ingredients: In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.
Mix the Wet Ingredients: In a separate bowl, whisk the egg, then add buttermilk, vegetable oil, and vanilla extract. Mix until smooth.
Combine Wet and Dry Ingredients: Make a well in the center of the dry ingredients and pour in the wet mixture. Stir gently until just combined (a few lumps are okay). Be careful not to overmix.
Fold in Blueberries: Gently fold in the blueberries into the batter.
Prepare the Crumble Topping: In a small bowl, mix the rolled oats, brown sugar, flour, cinnamon, and cold butter. Use your fingers or a fork to combine until it resembles coarse crumbs.
Fill the Muffin Tin: Divide the batter evenly into the prepared muffin cups, filling them about ¾ full. Sprinkle the crumble topping generously over each muffin.
Bake: Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve: Allow the muffins to cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely.
Notes
For a healthier option, you can substitute yogurt for buttermilk.