Preheat oven to 350°F with a rack in the center of the oven.
Prepare an 8×8″ baking pan by lightly spraying with cooking spray or greasing with butter. Line with parchment paper.
In a medium saucepan, melt butter over medium heat and whisk in granulated and brown sugars. Lower heat and stir in buttermilk and 1/4 cup of the chocolate chips. Remove from heat and whisk in the eggs and vanilla.
In a separate small bowl, whisk together the flour, cocoa powder, baking soda, baking powder and salt. Stir in the butter mixture until just combined.
Stir in the chocolate chips.
Pour into prepared brownie pan and spread evenly.
Bake for about 25 minutes or until it begins to pull from the edges of the pan. It will still be a little gooey in the center.
Remove from oven, cool completely (or at least until it is just lukewarm).
Slice into 16 squares and serve.
Vegetable oil can be substituted for butter in this recipe.