1. Preheat the oven to 375 degrees F. Grease a 12-cup muffin tin and set aside.

2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

3. In a large bowl, beat together the butter and sugar until light and creamy. Add the eggs, one at a time, beating well after each addition. Beat in the vanilla extract.

4. Add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

5. Gently fold in the buttermilk, blueberries, and walnuts.

6. Divide the batter evenly among the prepared muffin tin cups.

7. Bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

8. Let cool for 5 minutes in the tin before transferring to a wire rack to cool completely.