For high altitude adjustments, please see the instructions in the "Notes" section below!
Preheat oven to 350°F and line a 12 cup muffin tin with cupcake liners. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda and salt. Set aside.
In a large bowl, whisk together the oil, granulated sugar and brown sugar. Whisk in one egg at a time and add the vanilla extract.
In a measuring cup, measure out the buttermilk and then stir in the instant coffee/espresso powder.
Alternate mixing the dry ingredients and buttermilk mixture into the wet ingredients. Mix until just combined but do not overmix.
Fill each cupcake liner ⅔ full with the batter. Bake for 20 minutes in the oven, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let cupcakes cool in a pan for 10 minutes before transferring to a wire rack to cool completely. Frost with dark chocolate buttercream (recipe below).