Preheat the oven to 350 degrees. Line 24 muffin cups with paper liners.
Follow the cake mix instructions, putting all the ingredients in a large bowl and using buttermilk in place of the water specified on the box (the box will call for more water than how much buttermilk you need), using the amount of vegetable oil specified on the box (1/3 cup for white cake, 1/2 cup for chocolate), and adding the 4 large eggs. Beat with an electric mixer until moistened, about 30 seconds. Increase the speed to high and beat for 2 minutes.
Spoon half of the batter into a ziplock bag. Snip a 1/4-inch corner from the bag and fill the cupcake liners 2/3 of the way full.
Bake until golden and a toothpick inserted into the center comes out clean (15-20 minutes). Remove the cupcakes from the baking pans, place on a wire rack and let cool completely.