Add water to a medium sized saucepan and heat over medium-low heat. In a separate bowl, add in the semi-sweet chocolate. Place this bowl of the water and melt the chocolate. Stir continuously until all the chocolate melts. Then, remove from the heat and set aside.
Next, add the egg whites and granulated sugar to an electric mixer bowl. Add this mixer bowl over the same saucepan filled with water. Whisk constantly until all the sugar is dissolved. To check, rub the mixture between your finger-tips. If it feels like sand, the sugar is not fully dissolved yet. Continue to whisk.
Once the sugar dissolves and the texture looks like an opaque white remove from the heat. (If the mixer bowl is hot to the touch, allow to come back to room temperature before continuing).
Whisk the eggs whites and sugar on a medium-high speed until stiff peaks form. Once formed, reduce the speed to a medium-low.
Once stiffed peaks are formed, lower the speed to a medium-low. Then, add a tablespoon at a time of room temperature unsalted butter. Once the tablespoon is well mixed into the meringue, add the next tablespoon. Continue this process for the remaining butter. Then, turn up the mixer to a medium-high speed and fluffy up the buttercream. Next, add in the kosher salt, vanilla extract, and cream of tartar. Mix well. Lastly, fold in the melted chocolate. Now, it's time to frost that stunning yellow birthday cake.
Add a tablespoon at a time of room temperature butter. Wait until the tablespoon fully incorporates into the mixture before adding the next tablespoon.
Next, add the kosher salt, cream of tartar, and vanilla extract. If the buttercream seems loose, increase the speed to medium-high and allow to mix for 2 minutes.
Lastly, fold in the melted chocolate. Mix well. Then use as desired.