Step 1

In a 1-quart pot, bring heavy cream and milk to a simmer. Remove from heat and transfer mixture to a liquid measuring cup (if you have one) for easy pouring.

Step 2

In a small bowl, vigorously whisk together egg yolk, sugar, and vanilla for 30 seconds. Whisking continuously, very slowly drizzle in hot cream mixture, being careful not to go too fast and scramble the egg.

Step 3

Once cream is completely incorporated, wipe out your 1-quart pot and pour in egg and cream mixture. Heat over medium heat, stirring constantly until thickened and custard coats the back of your spoon, 1 to 2 minutes. Do not simmer. Set aside or store covered in the refrigerator if making in advance.

Step 4

Remove the top rack from your oven so your souffles can sit on the bottom rack and rise unobstructed. Preheat your oven to 400°F.

Step 5

Use a pastry brush and softened butter to grease the entire inside of the ramekins. Coat the sides in long brush strokes from the bottom up to the top all the way around. Add 1/2 tablespoon of the sugar to each ramekin and shake so that all of the butter is coated. Discard any loose sugar. Place the ramekins in the fridge.

Step 6

Place chocolate in your double boiler* and melt over barely simmering water. Remove from heat and stir in diced, room-temperature butter. Set aside.

Step 7

In a medium bowl, combine egg whites and cream of tartar.

Step 8

With a handheld electric mixer, beat whites until stiff peaks form--eggs should be glossy and when beaters are lifted straight out, peaks should stand straight up and not flop over at the tip. Place bowl in the refrigerator.

Step 9

Whisk the egg yolks into the chocolate mixture until smooth. Remove egg whites from the refrigerator and use a silicone spatula to fold about 1/4 of the whites into the chocolate mixture. Continue to fold the rest of the whites in in thirds until mixed. The mixture will not be completely uniform in color, but there should be no very distinct streaks of white. Divide the mixture between 2 ramekins, filling to the very top. Discard any extra. (If making ahead, skip to the notes.)

Step 10

Use a knife or dough scraper to scrape excess off the top for a perfectly flat top, then run the very tip of your index finger all the way around the inside of the ramekin, making a small channel between the souffle and the edge of the dish. Wipe edges and outsides of the ramekins with a clean kitchen towel and place on a baking sheet.

Step 11

Bake on the bottom rack of your oven for 11 to 13 minutes, until lifted nicely. Do not open your oven before the timer goes off as the cold air could affect the souffles' rise. Souffles will begin to slowly drop just a couple minutes after they come out of the oven, so serve immediately. At the table, top with creme anglaise and enjoy.