Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly. This prevents the muffins from sticking and makes for easy removal.

In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined. This ensures even distribution of the leavening agents.

In another bowl, combine the buttermilk, melted butter, eggs, and vanilla extract. Whisk until smooth. 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. Overmixing can lead to dense muffins.

Gently fold in the blueberries, being cautious not to break them to avoid turning the batter purple.

In a small bowl, combine the flour, brown sugar, cinnamon, and rolled oats (if using).

Add the cold, cubed butter and use your fingers or a fork to mix until the mixture resembles coarse crumbs. This will create a crunchy topping when baked.

Scoop the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the streusel topping generously over each muffin.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. The streusel should be golden brown.

Once baked, remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a cooling rack. This helps them set and makes them easier to remove.