Getting ready: Center a rack in the oven and preheat the oven to 400 degrees F. butter (or spray – it’s easier) 6 cups of a regular-size muffin pan, preferably a disposable aluminum foil pan, dust the insides with flour and tap out the excess. Put the muffin pan on a baking sheet.
Sift the flour, cocoa and salt together.
Set a heatproof bowl over a saucepan of gently simmering water, put the coarsely chopped chocolate and the butter in the bowl and stir occasionally over the simmering water just until they are melted – you don’t want them to get so hot that the butter separates. Remove the bowl from the pan of water.
In a large bowl, whisk the eggs and yolk until homogenous. Add the sugar and whisk until well blended, about 2 minutes. Add the dry ingredients and, still using the whisk, stir (don’t beat) them into the eggs. Little by little, and using a light hand, stir in the melted chocolate and butter. Divide the batter evenly among the muffin cups and sprinkle the finely chopped chocolate over the batter.
Bake the cakes for 13 minutes. Transfer them, still on the baking sheet, to a rack to cool for 3 minutes. (There is no way to test that these cakes are properly baked, because the inside remains liquid.)
Line a cutting board with a silicone baking mat or parchment or wax paper, and, after the 3-minute rest, unmold the cakes onto the board. Use a wide metal spatula to lift the cakes onto dessert plates.