Add eggs, vanilla, butter and sour cream (see note 1). Mix well.
Add chocolate chunks and mix (but don't overmix).
Pour batter (see note 2) in prepared muffin cups and bake for around 25 minutes, or until muffin tops are lightly golden brown.
Notes
Note 1: If you don't have sour cream at home you could use heavy cream and add the juice of 1/2 lime or small lemon (1 tablespoon) or even 1 tablespoon of apple vinegar (or any other white vinegar).Note 2: The batter should be firm. This is not a runny batter.Adapted from Recipe Encyclopedia.