Preheat your oven to 375°F (190°C). This ensures the muffins bake evenly and rise properly.

Grease a 12-cup muffin tin with cooking spray or line with paper liners. This prevents the muffins from sticking.

In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt. Whisking aerates the flour and ensures even distribution of the leavening agents.

In another bowl, combine the melted butter, buttermilk, eggs, and vanilla extract. Whisk until well combined. This mixture adds moisture and flavor to the muffins.

Pour the wet ingredients into the dry ingredients. Stir gently with a rubber spatula until just combined. Be careful not to overmix; a few lumps are okay. Overmixing can lead to tough muffins.

Gently fold in the blackberries, being careful not to break them apart. This ensures the muffins have whole berries in every bite.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full. This allows room for the muffins to rise without overflowing.

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

Remove the muffins from the oven and let them cool in the tin for 5 minutes. Then, transfer them to a cooling rack to cool completely. This prevents them from becoming soggy.

Enjoy your delicious blackberry muffins warm or at room temperature!