Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.

In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs one at a time, beating well after each addition.

Stir in the vanilla extract.

In another bowl, mix the flour, baking powder, and salt together.

Gradually add the dry ingredients to the wet mixture, alternating with the milk.

Fold in the chopped walnuts if using.

Divide the batter evenly among the prepared muffin cups.

Bake for 20-25 minutes, or until a toothpick inserted comes out clean.

Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack.