Preheat your oven to 350°F (180°C) and line a cupcake pan with liners. Preparing everything in advance helps the cupcakes bake evenly and smoothly.
Beat the butter and sugar together until light and fluffy. This step adds air to the batter, which helps create soft cupcakes.
Mix in the egg, egg white, and vanilla until just combined. Avoid overmixing to keep the batter light.
Alternate adding the dry ingredients and buttermilk to the batter. Start and end with dry ingredients, mixing gently until just combined.
Fill each liner about two-thirds full and bake for 15–20 minutes. Let the cupcakes cool completely before frosting.
Beat butter and cream cheese until smooth. Add cocoa powder, salt, and powdered sugar gradually until fluffy and creamy.
Pipe or spread frosting onto cooled cupcakes. Decorate as desired and enjoy.