Preheat oven to Preheat oven to 350°F. Line cupcake pan with cupcake liners.
Sift flour, baking soda and salt into a large bowl. (1 ⅓ cup cake flour, ¾ teaspoon baking soda, ¼ teaspoon salt)
Boil water and add espresso to it, stirring until dissolved. (⅓ cup water, ½ teaspoon instant espresso)
Whisk cocoa powder, then buttermilk and vanilla into water/espresso mixture. (⅓ cup cocoa powder, ½ cup buttermilk, 1 teaspoon vanilla))
Using a stand mixer, whisk sugar and butter until fluffy. About 10 minutes. (1 ¼ cup sugar, ½ cup unsalted butter)
Add each egg and the yolk, one at a time, into the butter mixture, letting each one fully mix into the batter before adding the next. About 2-3 minutes of mixing in between each egg. Add in oil and mix until fully combined. (2 extra large eggs, 1 extra large yolk)
Fold in ½ of the flour mixture and then fold in ½ of the buttermilk mixture to the batter. Then fold in remaining flour and then remaining buttermilk mixture.
Bake for 20-25 minutes or until a toothpick inserted comes out clean.