1cupsemi-sweet chocolate chipsI like a mix of standard and mini size, about 3/4 cup standard and 1/4 cup mini measured into the same measuring cup
Preheat the oven to 400ºF. Spray the wells of a 6 count jumbo muffin tin (or line with jumbo liners) generously with non-stick spray. Set aside.
In a medium size bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, whisk together the cooled butter, vegetable oil, granulated sugar, brown sugar until smooth. Whisk in the eggs, vanilla, and buttermilk until everything is thoroughly combined.
Gently fold the dry ingredients into the wet ingredients. When the batter is mostly combined, add in the chocolate chips and continue stirring gently until everything is incorporated, being careful not to overmix.
Spoon the batter evenly into the 6 wells. Two ice cream scoop fulls works best, totaling about 1/2 cup of batter in each well. Sprinkle with additional chocolate chips, if desired.
Bake the muffins for 5 minutes, then reduce the oven temperature to 350ºF without opening the door, and bake for another 15-20 minutes. Muffins are done when a toothpick inserted into the center comes out mostly clean and the tops are golden brown. Remove from the oven and allow them to cool in the pan for about 10 minutes before removing from the pan to cool completely on a wire rack. Muffins stay fresh covered at room temperature up to 5 days or in the refrigerator up to 1 weeks. Muffins freeze well, up to 2 months. Thaw in refrigerator or heat in the microwave about 15 seconds before serving.
*If you don't have buttermilk on hand, you can make your own: see my post about homemade buttermilk.