Preheat your oven to 375°F (190°C). This ensures that the muffins bake evenly and rise properly.
In a large mixing bowl, whisk together the flour, 3/4 cup of granulated sugar, baking powder, baking soda, and salt. This helps to evenly distribute the leavening agents and sugar.
In another bowl, whisk together the buttermilk, melted butter, eggs, and vanilla extract until well combined. The buttermilk adds moisture and a slight tang to the muffins.
Pour the wet ingredients into the dry ingredients and gently fold them together using a rubber spatula. Be careful not to overmix; a few lumps are okay.
Gently fold in the blueberries, ensuring they are evenly distributed throughout the batter.
Make the Crumble Topping
In a medium bowl, combine the flour, brown sugar, rolled oats, and cinnamon. Mix well.
Add the cold, cubed butter to the mixture. Using your fingers or a pastry cutter, work the butter into the dry ingredients until the mixture resembles coarse crumbs. This will create a crunchy topping.
Bake the Muffins
Line a muffin tin with paper liners or grease it with cooking spray. This prevents the muffins from sticking.
Spoon the muffin batter into each muffin cup, filling them about 2/3 full. This allows room for the muffins to rise.
Sprinkle the crumble topping generously over each muffin. This adds a delicious crunch.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. This ensures that the muffins are fully cooked.
Once baked, remove the muffins from the oven and let them cool in the pan for 5 minutes before transferring them to a cooling rack.
Notes
These muffins can be stored in an airtight container at room temperature for up to 3 days or frozen for up to 3 months. To reheat, simply warm in the microwave for a few seconds.