First of all, pour the boiling coffee decoction over the chocolate and stir until it has melted. Set this aside.
I use filter coffee decoction because it is always available in my kitchen and the aroma of that coffee is something I love. If you do not have this you can use espresso or mix up some powdered instant coffee in boiling water and use that instead.
Using an electric mixer, beat the butter and sugar until it is light and fluffy. Add the egg yolk and beat well till mixed. Now add the vanilla and coffee-melted chocolate and mix well.
Whisk together the flour, cocoa powder, baking soda, and salt. Stir in half of the dry ingredients into the above mixture, add the yogurt and mix till blended. Now add the remaining dry ingredients. Mix again till just blended.
Whip the egg whites in a clean and dry bowl, till stiff. First fold a third of this into the chocolate batter, and then the rest till there are no streaks of white remaining. Do not overfold.
Equally divide the batter between 12 paper lined muffin cups. Bake at 180C (350F) for about 25 minutes, till the batter seems just set in the center. Remove the cupcakes from the oven, cool for a few minutes, and then remove the cupcakes from the muffin tins. Cool completely on a wire rack before frosting them.