Center a rack in the oven and preheat the oven to 400 degrees F. Grease (using butter or non-stick cooking spray) 6 cups of a regular-size muffin pan, dust the insides with flour (or cocoa powder!) 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 4 ounces of the finely chopped chocolate and the butter in the bowl and stir occasionally over the simmering water just until they are melted. 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 remaining 1 ounce of finely chopped chocolate evenly 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.
Notes
To freeze, wrap individually in plastic wrap and store in a ziploc freezer bag for up to 2 months. Microwave for 30 to 45 seconds before serving.
While I haven't done so, I think you could easily scale this recipe up to make more than 6 cakes.