Preheat your oven to 350°F (175°C). Line a cupcake pan with cupcake liners to prepare for baking.
In a large mixing bowl, whisk together the cocoa powder, flour, baking soda, baking powder, and salt to evenly distribute the dry ingredients.
In another bowl, combine the eggs, sugar, brown sugar, vegetable oil, vanilla extract, coffee extract, and buttermilk. Whisk until smooth.
Gradually add the wet mixture to the dry ingredients, stirring gently until just combined. Do not overmix; a few lumps are fine.
Fill each cupcake liner about two-thirds full with batter using a ladle or scoop.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Remove cupcakes from oven and cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
For the frosting, beat softened butter in a large bowl until creamy. Gradually add powdered sugar on low speed until combined.
Add vanilla extract and heavy cream, then beat on high until the frosting is fluffy and smooth.
Once cupcakes are fully cooled, frost generously with the vanilla buttercream using a spatula or piping bag.