Put a couple of inches of water in a medium sauce pan over medium heat and place a glass or metal bowl over the pan. Do not let the water touch the bottom of the bowl.
Add the butter, chocolate, milk, and powdered sugar to the bowl and stir occasionally until everything is melted. Remove the bowl from the heat.
In a mixing bowl, whisk together the egg, yolk, sugar, vanilla and salt until it forms a thick foam and falls in a ribbon. If you use a mixer, it will take about three minutes. I whisked it by hand. It's tiring but completely achievable.
Gently fold in a tablespoon of flour, then fold in the chocolate mixture. You want to work as lightly as you can since all the leavening will come from your foam. Keep folding until the streaks have almost disappeared.
Divide the batter between your two waiting ramekins and pop them in the oven for 10 to 12 minutes. It should be just set on the top.
Allow to cool for about two minutes. Run a sharp knife around the edge of each cake and turn them out onto your plate. You can garnish with some powdered sugar, chocolate syrup, whipped cream, or even ice cream. This cake is completely awesome all by itself, and you will be sad when it's gone.