Preheat oven to 325° F. Line a cupcake pan with cupcake liners, set aside.
In a medium bowl, whisk the flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixture fitted with a paddle attachment, cream together the butter and brown sugar. Add the oil and vanilla and mix. Add the egg and egg yolk and mix to combine.
With the mixture on low speed, add half of the flour mixture, all of the buttermilk, then the rest of the flour mixture. Mix until partially combined, then fold in the chocolate chips but don’t overmix.
Fill the cupcake cavities half to 2/3's full, using approximately 2 ½ - 3 T of batter in each compartment.
Bake for 17-20 min, or until a toothpick inserted into the middle come out clean or with a couple crumbs stuck to it.
Remove from oven, let cool in pan for 5 mins then turn out to cool on a wire rack until room temperature.