For high altitude instructions, see "Notes" section below.
Preheat oven to 350°F. Line a 12 cup muffin pan with cupcake liners. Line a second pan with 6 more liners (recipe makes about 18 cupcakes). Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the butter on medium-high speed until smooth and creamy (about 1 minute). Add the sugar and beat on medium-high speed until light and fluffy (about 2 minutes).
Add the egg whites, vanilla extract and vanilla bean paste. Beat on medium speed until combined.
Beat in the sour cream on medium speed. Scrape down the sides of the bowl with a spatula as needed.
With the mixer on low speed, slowly add the dry ingredients until just combined. With the mixer still on low speed, slowly pour in the milk and mix until combined. Do not overmix.
Pour in the mini chocolate chips and fold into the batter using a spatula. Do not overmix.
Spoon the batter into the cupcake liners until they are filled about 2/3 full (I like to use a cookie scoop for this!)
Bake for 18-22 minutes, or until the centers of the cupcakes spring back when touched. Allow cupcakes to cool completely before frosting.