Preheat oven to 350°F and line a standard muffin tin with 12 muffin liners.

Prepare the streusel. In a small bowl, combine the flour, oats, brown sugar, pecans and salt. Cut the butter into small cubes and add to the flour mixture. Use a pastry cutter or your hands to incorporate the butter until the mixture resembles coarse crumbs.

In a medium bowl, whisk together the melted and cooled butter, sugar, eggs, buttermilk and vanilla extract until well combined.

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

Add the wet ingredients to the dry ingredients. Fold until just combined.

Gently fold in the blueberries.

Scoop the batter into the prepared muffin tin. Stagger the muffin batter so the muffin tops have more room to expand. Sprinkle the streusel on the top of each muffin.

Bake for 30-35 minutes. The tops will be golden brown and an inserted toothpick should come out clean.

Allow the muffins to sit in the tin for 20 minutes and then remove to a wire rack to cool completely.