1. In a clean, grease-free bowl, combine the egg whites and sugar. Place the bowl over a saucepan with gently simmering water (bain-marie), making sure the bottom doesn’t touch the water. Heat the mixture while whisking constantly until it reaches 70°C. If you don’t have a thermometer, test by rubbing a small amount between your fingers — it should feel completely smooth, with no sugar grains remaining.
2. Once the sugar is dissolved, transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Whip on high speed for 10–15 minutes until stiff, glossy peaks form and the bowl is cool to the touch. The meringue must be at room temperature before you move on to the next step.
3. Switch to medium speed and gradually add the butter, one cube at a time. Make sure the butter is soft but not melted. The mixture may look curdled or soupy at some point, that’s completely normal. Just keep mixing and it will come together. 💡 If the buttercream stays loose or too soft, place the bowl in the fridge for 15–20 minutes and whip again. You can repeat this if needed until the texture is stable and fluffy. Once the butter is fully incorporated, scrape down the sides and bottom of the bowl to make sure everything is evenly mixed.
4. Meanwhile, melt the dark chocolate using a bain-marie or in short bursts in the microwave. Let it cool slightly, it should be fluid but not hot when added.
5. Replace the whisk attachment with the paddle attachment and pour the melted chocolate into the buttercream. Mix on the lowest speed for at least 15 minutes. This helps knock out excess air and gives your buttercream a beautifully smooth, dense texture that’s perfect for piping or frosting cakes.

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