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6 ounces egg whites (⅔ cup; 170g), from 5 to 6 large eggs

11 ounces toasted sugar, Belgian cassonade, or light brown sugar (about 1 ⅔ cups; 310g)

1 teaspoon (4g) Diamond Crystal kosher salt (plus more if needed)

¼ teaspoon cream of tartar

16 ounces unsalted butter (4 sticks; 455g), softened to about 65°F (18°C)

1 teaspoon (5g) vanilla extract (plus more if needed)

8 to 14 ounces finely chopped dark chocolate, between 70 and 77% cocoa solids (about 1 ⅓ to 2 ⅓ cups; 225 to 395g)

1 teaspoon instant espresso powder (optional), dissolved in a few drops of hot water

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1. Fill a wide pot with at least 1 ½ inches water, with a thick ring of crumpled tinfoil placed on the bottom to act as a 'booster seat' that will prevent the bowl from touching the bottom of the pot.

2. Place over high heat until steaming-hot, then adjust temperature to maintain a gentle simmer.

3. Combine egg whites, sugar, salt, and cream of tartar in the bowl of a stand mixer.

4. Set over steaming water, stirring and scraping constantly with a flexible spatula, until egg whites hold steady at 185°F (85°C).

5. Transfer to a stand mixer fitted with a whisk attachment and whip at high speed for about 10 minutes, until meringue is glossy, stiff, and cool to the touch, around 90°F (32°C).

6. With mixer still running, add butter, 1 or 2 tablespoons at a time.

7. Mix in vanilla extract on low speed until well combined.

8. Melt chocolate, either in a microwave or over a water bath, until fluid and warm.

9. Scrape all of the warm chocolate into stand mixer bowl at once, then immediately begin whipping on medium-high until fully incorporated.

10. Adjust to taste with additional salt, vanilla, or dissolved espresso powder to add depth of flavor.

11. Use buttercream right away, or transfer to a large zipper-lock bag, press out air, and seal.

Notes

Sugars with a caramel flavor, such as quick- or slow-roasted sugar, or even Belgian cassonade (candi) sugar, tame the sweetness of meringue while underscoring the chocolate with a rich, toasty note. Molasses-based alternatives, such as turbinado, Demerara, or American-style light brown sugar, work well, too, but will give the buttercream a tangier profile overall.

Make Ahead and Storage

Buttercream can be refrigerated for up to 2 weeks and frozen for up to several months. (The main issue with longer storage in the freezer is odor absorption, not spoilage.) Rewarm to approximately 73°F (23°C) and re-whip before using.