Heat oven to 350°F. Grease a 9×13-inch pan and line with parchment.
Whisk flour, baking powder, baking soda, and salt in a large bowl.
Combine melted butter, oil, granulated sugar, and brown sugar until glossy.
Beat in eggs one at a time, then stir in vanilla.
Whisk in sour cream and buttermilk until smooth.
Fold wet mixture into dry ingredients just until no dry streaks remain.
Gently fold in mini chocolate chips.
Spread batter in pan, smooth the top, and sprinkle with topping chips.
Bake 24–28 minutes, until golden and a toothpick comes out with a few moist crumbs.
Cool in the pan at least 20 minutes. Slice and serve warm or at room temperature.