Preheat oven to 350F.
Line one 12 cups standard muffin tin or two 24-cup mini-muffin tins with cupcake liners. I used a 12 cups standard muffin tin, plus a 6 cups one.
In a medium bowl mix dry ingredients: flour, sugar, baking powder and salt.
In the large bowl of an electric mixer, whisk softened butter on medium speed for 1-2 minutes.
Add dry ingredients and combine with butter for 30-40 seconds.
Add milk, oil, mint extract, and eggs one at a time, mix on medium-high for one minute. Make sure to scrape the sides of the bowl occasionally.
Using a spatula, stir in the chocolate chips.
Divide the batter evenly in the prepared tin. Bake for 18-23 minutes, or until a tester inserted in the center of the cupcakes comes out clean and rotate the tin about halfway through baking. Bake mini cupcakes for 11-13 minutes.
Cool the cupcakes on a rack in the tin for 10 minutes, and then remove from the tin and let them cool completely on the rack.