Begin by preparing the ramekins by brushing them with butter in a vertical direction.
Then, coat the butter with a thin layer of granulated sugar. Set them aside.
Preheat your oven to 360°F (182°C)
In a heavy bottom potbelly over medium heat, bring the milk to a boil.
Next, in a separate bowl, combine the egg yolk and cornstarch by whisking it until the mixture is smooth and the cornstarch is dissolved. Really whisk it together so you don't get any clumps of cornstarch.
Add about ⅓ cup of the warm milk to the egg yolks and mix. This is tempering the eggs. Once you've done this, pour the egg/milk mixture back into the pot with the rest of the hot milk.
Stir it constantly over medium heat to cook the egg/milk mix to a boil. Be careful not to cook the eggs too quickly.
Remove the pot from heat, and then add the chocolate to the pot.
Keep stirring the mixture until the chocolate is fully incorporated.
Then, transfer the chocolate mixture to a bowl and set it aside.
Next, in the bowl of your stand mixer, add in your egg whites and a pinch of cream of tartar. Make sure your bowl is very clean, use a little bit of white vinegar to clean your bowl and whisk attachment if needed.
Begin to whip the egg whites in the bowl of your stand mixer on medium speed while bringing them to a soft peak.
Then, slowly add in the sugar while mixing.
Continue to whip the egg whites at medium speed until they have reached a smooth medium peak.
Then, once the egg whites have a smooth medium peak, add ⅓ of the egg whites to the chocolate pastry cream and mix them together until they are fully incorporated.
Now, pour the chocolate mixture into the bowl with the rest of the whipped egg whites.
Next, gently fold the egg whites and chocolate mixture together carefully. Try not to deflate the egg whites too much. You only want to fold them until they are fully incorporated.