Center a rack in the oven and preheat the oven to 350 degrees F. Fit the 12 molds in a regular-size muffin pan with paper muffin cups, or butter them, dust them with flour and tap out the excess.
Whisk together the flour, cocoa, baking powder, baking soda and salt.
Working with a stand mixer, preferably fitted with the paddle attachment, or with a hand mixer in a large bowl, beat the butter at medium speed until soft and creamy. Add the sugar and beat for about 2 minutes, until it is blended into the butter. Add the egg, then the yolk, beating 1 minute after each addition and scraping down the sides and bottom of the bowl with a rubber spatula as needed. Beat in the vanilla, then reduce the mixer speed to low and add half the dry ingredients, mixing only until they disappear. Scrape down the bowl and add the buttermilk, mixing until incorporated, then mix in the remaining dry ingredients. Scrape down the bowl, add the melted chocolate and mix it in with the rubber spatula. Divide the batter evenly among the muffin molds.
Bake for 22 to 25 minutes, or until the tops of the cakes are dry and springy to the touch and a knife inserted into their centers comes out clean. Transfer the muffin pan to a rack and let the cakes cool for 5 minutes before unmolding. Cool to room temperature on the rack before glazing.
To make the Glaze
Melt the chocolate in a heatproof bowl over a saucepan of simmering water. Transfer the bowl to the counter and let stand for 5 minutes.
Using a small whisk or rubber spatula, stir the confectioners’ sugar into the chocolate, followed by the pieces of cold butter. The glaze may be very thin at this point or may be perfectly spreadable. If it is too thin to spread or use as a dip, stir it over ice water for a few seconds — really less than a minute. With a small metal icing spatula, give each cupcake a crown of shiny ganache, and let the glaze set at room temperature (or in the fridge if you are in a hurry).
This recipe was submitted by one of our readers and has not been tested by our food editors. We are not responsible for errors in this recipe, but if you find one, please let us know.