Line 3 baking sheets with parchment paper and preheat the oven to 350 F.
Sift together the flour, powder, and salt onto parchment paper.Beat together the butter, shortening, and sugars in the bowl of a stand mixer with a paddle attachment on medium speed until light and fluffy. Add in the eggs and buttermilk and beat together.
Mix the milk, soda, and vinegar together and then add to the batter. Add in the vanilla and beat for 2 minutes on medium so that everything is well combined.
Use a rubber spatula to mix in the mini chocolate chips.
Use a 2 tablespoon cookie scoop to portion the batter on the baking sheets. I was able to fit 10-12 whoppies per baking sheet. Leave space for the whoopies to spread slightly. (please ignore the creepy hand shadows)
Bake for 10 minutes or until the edges just begin to brown.
Transfer to a cooling rack.