1. Preheat the oven to 350ºF (180ºC). Line a muffin tin with 12 cupcake liners.
2. Pour the boiling water or coffee over the chocolate, and stir until melted. Set aside.
3. In the bowl of a stand mixer, or by hand, beat the butter and sugar until light and fluffy.
4. Add the egg yolks and mix until thoroughly incorporated. Then mix in the vanilla and the melted chocolate.
5. Whisk together the cake flour, cocoa powder, baking soda, and salt. Stir in half of the dry ingredients, then add the buttermilk or sour cream, then the remaining dry ingredients, mixing just until blended.
6. In a clean, dry bowl, whip the two egg whites until stiff, then fold one-third of them in to the chocolate batter, then the rest. Fold just into there are no streaks of white remaining, but don’t overfold.
7. Divide the batter between the muffin cups and bake for about 25 minutes, until the batter feels just set in the center. Remove from the oven, then let cool for a few minutes.
Once cool enough to handle, remove the cupcakes from the muffin tin and let cool on a wire rack completely before frosting.