Preheat oven to 350 F. and set oven rack to middle position.
Spray 12-cup muffin pan with nonstick cooking spray (or use Pam), or line with cupcake liners.
In medium bowl, combine flour, baking powder, baking soda, and salt.
In large bowl, whisk together melted butter, sugar, brown sugar, and vanilla.
Whisk in eggs, followed by sour cream.
Add dry ingredients and whisk until combined.
Stir in 2/3 c of choc. chips.
Fill muffin cups about 2/3 full.
Sprinkle tops with about 1/3 c choc. chips.
Bake 18-20 min. at 350 F.
Let muffins cool in pan 10 min.; then remove to rack or plate.
Muffins can be re-heated in microwave 15-20 seconds.