Preheat the oven to 350°F (177°C). Line a muffin pan with cupcake liners. Line a second pan with 2 liners—this recipe makes about 14 cupcakes. Set aside.

Whisk the flour, cocoa powder, espresso powder, baking powder, baking soda, and salt together in a large bowl until thoroughly combined. Set aside. In a medium bowl, whisk the eggs, granulated sugar, brown sugar, oil, and vanilla together until completely smooth. Pour half of the wet ingredients into the dry ingredients. Then half of the buttermilk. Gently whisk for a few seconds. Repeat with the remaining wet ingredients and buttermilk. Stir until just combined; do not overmix. The batter will be thin.

Pour or spoon the batter into the liners. Fill only halfway to avoid spilling over or sinking. Bake for 18-21 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool completely before frosting.

Frost cooled cupcakes with chocolate buttercream. You can swipe the frosting with an icing knife or use a piping tip. Leftover cupcakes keep well covered tightly in the refrigerator for 3 days.