Preheat your oven to 350°F (175°C). Line a cupcake pan with cupcake liners and set aside.
In a large mixing bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking powder, baking soda, and salt.
In another bowl, combine the buttermilk, vanilla extract, and instant coffee granules if using. Mix until well combined.
In a large mixing bowl, beat together the granulated sugar and vegetable oil until well blended. This might take about 2 minutes using an electric mixer.
Add the eggs and egg white to the sugar-oil mixture, one at a time, mixing well after each addition.
Gradually add the dry ingredient mixture into the wet ingredients, alternating with the buttermilk mixture. Start and end with the dry ingredients. Mix until just combined.
Using a scoop or spoon, fill each cupcake liner about ¾ full with batter.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cupcakes from the oven and let them cool in the pan for about 5 minutes. Then, transfer them to a wire cooling rack to cool completely.
In a mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar, cocoa powder, and salt, mixing on low speed until combined. Then add the vanilla extract and heavy cream. Beat on high until the frosting is light and fluffy.
Once the cupcakes are completely cool, use a spatula or piping bag to frost each cupcake with the buttercream frosting.
