Chocolate Swiss Buttercream

Ingredients

• 6 ounces egg whites, from 5 to 6 large eggs
• 11 ounces toasted sugar, Belgian cassonade, or light brown sugar
• 1 teaspoon Diamond Crystal kosher salt; for iodized salt, use half as much by volume or the same weight
• 1/4 teaspoon cream of tartar
• 16 ounces unsalted butter, softened to about 65°F
• 1 teaspoon vanilla extract
• 8 to 14 ounces finely chopped dark chocolate, between 70 and 77% cocoa solids
• 1 teaspoon instant espresso powder, optional, dissolved in a few drops of hot water

Directions

1. Add at least 1 1/2 inches of water to a wide pot, and set a sturdy ring of crumpled tinfoil in the base so the mixer bowl can sit above the pot instead of resting on it. Heat the pot over high until the water is very steamy, then lower the heat enough to keep it at a gentle simmer. In the bowl of a stand mixer, combine the egg whites, sugar, salt, and cream of tartar. Place the bowl over the steam, stirring and scraping the whole time with a flexible spatula, until the mixture reaches and holds 185 degrees F (85 degrees C). This should take just 10 to 12 minutes, so raise the heat if it is dragging. Move the bowl to the stand mixer, attach the whisk, and beat on high for about 10 minutes, until the meringue is shiny, stiff, and cool when you touch it, around 90 degrees F (32 degrees C).
2. Leave the mixer running and add the butter in 1 or 2 tablespoon additions. The meringue will lose a lot of its volume at first, and it may look loose or even soupy for a bit, but the cool butter will bring the mixture down in temperature and help it thicken. When it is finished, the buttercream should be thick, smooth, and soft without being runny, around 72 degrees F (22 degrees C). Add the vanilla extract and mix on low until it disappears into the buttercream.
3. Melt the chocolate in the microwave or over a water bath until it is warm and fluid. Use the lower end of the suggested quantity range for a gentler, “milk” chocolate effect, or the full amount for the strongest chocolate flavor. Scrape all of the warm chocolate into the mixer bowl in one addition, then whip right away on medium-high until it is completely blended in. Scrape the bowl with a flexible spatula so no streaks of chocolate are hiding along the sides, then keep mixing until the buttercream is even throughout. If desired, fine-tune the flavor with more salt, more vanilla, or dissolved espresso powder.
4. Use the buttercream immediately, or move it to a large zipper-lock bag, squeeze out the air, and seal it. The buttercream can be refrigerated for up to 2 weeks or frozen for up to several months. With longer freezer storage, the bigger concern is picking up odors, not spoilage. Before using, warm it back to approximately 73 degrees F (23 degrees C) and whip it again.
5. Troubleshooting: Above 76 degrees F (24 degrees C), the buttercream will turn loose and soft; chill it for 15 minutes, then whip again so it can cool and thicken. Below 70 degrees F (21 degrees C), it will be firm and heavy, which makes it harder to spread on cakes and slower to melt on your tongue, giving it a greasy feel. To warm it, set the bowl briefly over steaming water, only until the edges just begin to melt, then whip again until smooth. For more help, use the full troubleshooting guide and video.