Preheat oven to 350 degrees F. Grease 18 muffins cups or line with paper liners.
In a large mixing bowl, sift together the flour, baking powder, baking soda and salt; mix well.
In a medium mixing bowl, whisk together the sugar, butter, milk, vanilla, sour cream, eggs and yolk.
Pour the wet ingredients into the dry and fold gently with a rubber spatula until the dry ingredients are mostly moistened. The batter will be lumpy and there will be streaks of flour. Choose an add-in. I used blueberries, but you can use a cup and half of other types of frozen fruit. Fold add-in into batter and at this point, make sure all the flour in incorporated.
Using a measuring spoon for scooping, scoop batter into tins. It should be slightly stiff and go up and over the top by about 3/4 inch. Bake for 30-35 minutes. Let the tin cool on a rack for 15 to 20 minutes.
Makes 18 muffins (or 12 large bakery style muffins)