Preparation

• Put a metal or Pyrex pie plate or cake pan in the freezer to chill. Lightly butter six 6-oz. ramekins or custard cups. Coat with sugar and tap out the excess.

Make the sauce:

• In a small bowl, combine the espresso powder with 2 Tbs. warm water and stir to dissolve.
• In a medium heatproof bowl set in or over a skillet of barely simmering water, melt the chocolate and butter, stirring frequently, until smooth. Add two pinches of salt, stir, and remove from the heat. Transfer 5 Tbs. of the chocolate mixture to the espresso and stir to blend. (Set the remaining melted chocolate aside.) Use a spatula to scrape the espresso mixture into a puddle on the chilled pie plate or cake pan and return to the freezer until firm, about 10 minutes. When the espresso-chocolate mixture is firm, use a teaspoon to scrape it into six rough balls. Keep the balls on the plate and refrigerate until ready to use.

Make the soufflé cakes:

• Reheat the remaining chocolate mixture by setting its bowl in or over the skillet of hot water. When it’s warm, remove from the heat and whisk in the cocoa and the 2 egg yolks.
• In a clean, dry bowl, beat the 3 egg whites and cream of tartar on medium speed in a stand mixer (or on high speed with a hand-held mixer) until the whites mound gently. Gradually beat in the sugar and beat until the whites form medium-stiff peaks when you lift the beaters; the tips should curl over but still look moist, glossy, and flexible. With a rubber spatula, fold about one-quarter of the egg whites into the chocolate to lighten it. Scrape the remaining whites into the bowl and gently fold in until blended, taking care not to deflate the whites. Take the chocolate balls out of the refrigerator and put one ball in the center of each ramekin. Divide the batter evenly among the ramekins and level the tops gently with the back of a spoon.
• Position a rack in the lower third of the oven; heat the oven to 400°F. Put the ramekins on a baking sheet. Bake until the soufflé cakes are puffed and possibly a little cracked on top (a toothpick inserted in the center will meet no resistance and emerge mostly clean—the tip will be wet from the sauce at the bottom), 11 to 14 min. Let cool for a few minutes before serving.

Make Ahead Tips

The assembled, unbaked soufflé cakes can be wrapped in plastic and refrigerated for up to two days. They do not need to come to room temperature before baking, but baking time will be one or two minutes longer.