Grease a 4 ounce canning jar or other small oven-safe jar or ramekin, place it in a slightly larger heat-safe ramekin or dish, and set it aside.
Place the chopped chocolate in a small, heat-safe bowl and melt it over a double boiler or in the microwave in 30-second bursts at 70% power.
Separate one of the eggs, and discard the yolk or use it for another purpose. Place the egg and egg white in a small, heat-safe bowl and beat to combine.
Pour the melted chocolate into the beaten eggs in a slow trickle to temper the eggs, beating continuously to combine.
Add the sugar, and mix to combine. The batter will be thickly pourable.
Pour the batter into the prepared canning jar, and shake back and forth gently to distribute into an even layer.
Pour enough room temperature water into the ramekin or dish that the canning jar is sitting in so that the water level reaches about 1 1/2 inches up the side of the jar.
Place in the center of the preheated oven and bake for 17 minutes, or until the top is domed and set.
For a more solid cake, bake for about 5 minutes more.
Remove the pan and jar from the oven, remove the jar from the water bath carefully with tongs, and allow the cake to cool briefly before serving.
Microwave instructions.
Grease a large, microwave-safe mug with cooking oil spray, and set it aside.
Place the chopped chocolate in a small, heat-safe bowl and melt it in the microwave in 30-second bursts at 70% power.
Add the granulated sugar, and whisk to combine well.
Once the melted chocolate has cooled until it’s no longer hot to the touch, add the egg and whisk to combine well. The batter will be thickly pourable.
Pour the batter into the prepared mug.
Place the mug in your microwave oven and cook at about 850 watts (See Recipe Notes) for 1 minute 15 seconds or until the cake is mostly set.
It’s fine if the cake seems a bit wet in the center, which is preferable to overcooking it.
The cake will likely rise relatively high and fall twice during cooking.
Allow it to cool briefly before serving with whipped cream and berries, or plain.
Notes
About microwave wattage.It is important to be able to approximate 850 or 900 watts of power in your microwave oven. My microwave is 1200 watts, so I set it to 70% power for this recipe.