Line 18 cupcake tins with liners. Preheat oven to 350 degrees F.
In a large bowl, whisk together the flour, baking powder and salt. In a large measuring cup, measure out the buttermilk and whisk in the egg whites and vanilla extract.
In the bowl of a stand-up mixer, add the butter, oil and sugar. Beat until light and fluffy about 2 minutes.
Alternating between the buttermilk mixture and the dry mix, add them to the butter mixture, until combined.
Remove 1 1/2 cups of batter and pour that into medium bowl and set aside (this is going to be the vanilla flavored). To the batter in the stand-up mixer, pour in the espresso and mix until just combined, about 15 seconds.
Add two tablespoons of the espresso-flavored cake batter to each of the cupcake liners. And then top each cupcake with a tablespoon of the vanilla-flavored batter. Using a teaspoon, smooth out any messy cupcakes. If they’re a little messy, it’s totally fine—they bake up kind of marbled-looking and it’s great.
Transfer to the oven to bake 18 to 20 minutes, until a skewer inserted into the center comes out clean.