Preheat the oven to 375 degrees F.
Fill two mini muffin pans with mini muffin liners. (If you only have one pan, just fill it and bake in batches.) Additionally, spray the liners with non-stick baking spray. (If making into standard-sized muffins, use a standard-sized muffin or cupcake pan.)
Add the dry ingredients (flour, baking powder, baking soda, sugars and salt) to a large mixing bowl and whisk well. Set aside.
In another mixing bowl, add the liquid ingredients (egg, vanilla, sour cream, milk, butter that's been melted and cooled down, vegetable oil) and whisk well. (If the liquid ingredients are cold, they may not mix well with the melted butter, which is why it's better to allow them to come closer to room temperature.)
Add the liquid ingredient mixture to the dry ingredient mixture and mix with an electric mixer on medium speed only until combined. (You can also mix with a whisk if you prefer.) Be very careful not to mix for too long. Only mix until the large lumps are gone. There may still be smaller lumps and that's okay. Mixing too long will cause dense and rubbery muffins. Note that the batter will be thick.
Fold in the mini chocolate chips with a large spoon or silicone spatula. (If you only have full-sized chocolate chips, try to chop them smaller.)
Add the batter to the mini muffin liners almost to the top. It's easier to use two teaspoons to spoon the batter into the mini cups. If you only have one pan and have leftover batter, cover it and chill it until the first batch bakes then bake the next batch.
Bake for approximately 11-13 minutes. (If baking in a standard-size muffin pan, bake for 16-18 minutes.) The muffins will puff up and a toothpick inserted into a muffin will come out clean or with a few moist crumbs on it, but no raw batter. The tops will also bounce back when touched. Be careful not to overbake or the muffins could turn out dry.
Remove the mini muffins from the oven and set the pan onto a cooling rack for about 5 minutes, then transfer the muffins directly onto the cooling rack and cool completely.
See the notes section for best ways to store these, serve them and for more questions.