Pre-heat the oven to 375 degrees F. Line a 12-cup standard muffin tray with cupcake liners or coat with non-stick cooking spray. In the bowl of a standing mixer fitted with a paddle attachment, cream the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Add the vanilla extract and beat again.
In a medium bowl sift together the flour, baking powder and salt. Add the dry ingredients to the creamed butter in batches, alternating with the milk. Carefully fold in the blueberries and walnuts, if using.
Use an ice cream scoop or spoon to evenly fill the muffin tray cups with batter. (Optional: Add a few plump blueberries on top of each muffin.) Finish by sprinkling the muffins with 3 teaspoons of sugar. Bake the muffins for about 25-30 minutes until lightly golden brown or until a wooden skewer comes out with scant dry crumbs. Remove the muffins from the tray after about 5 minutes and let them cool on a rack for at least 30 minutes.