To make the buttercream, combine granulated sugar (1) and water in a small saucepan and heat over medium heat until the sugar dissolves. Increase the heat and bring to a boil, then continue cooking until the syrup reaches 110°C (230°F).
While the syrup is cooking, whisk egg whites in a stand mixer until they are frothy. Add granulated sugar (2) and continue whisking until stiff peaks form.
Once the syrup reaches 120°C (248°F), pour it slowly into the egg whites, while continuing to whisk on medium-high speed.
Keep whisking until the meringue has cooled down to room temperature.
Begin adding butter, one tablespoon at a time, making sure each tablespoon is fully incorporated before adding the next one. Continue whisking until the buttercream is smooth and fully combined.
Melt the dark chocolate and add it to the buttercream, along with salt to taste. Whisk until fully combined.