In a heatproof bowl, whisk together the egg whites, sugar, lemon juice and salt. Place over a pot with boiling water and cook for about 10 minutes, until you can pinch it and can't detect any sugar granules (about 185 F).

Pour mixture into a stand mixer and whisk on high for about 10 minutes, until it becomes thick, pale and glossy. It'll take up to ten minutes to get to stiff peaks.

Add the butter slowly, no more than a tablespoonful at a time, allowing each to be incorporated before adding another. It might start to look separated but just keep whisking until you see it become smooth (stop the mixer at any point to test it by running your finger over it).

Add vanilla and mix. Then pour in the melted chocolate and whisk on medium until it all comes together. Because of the warm chocolate the buttercream may seem not solid enough but it will firm up in the fridge and at room temp, holds up just fine.