In a medium bowl place the egg whites, sugar, salt and place the bowl on a saucepan filled with water (double boiler)
Whisk lightly until mix well. whisk the mixture regularly to avoid it from burning. Place a thermometer on the mixture and make sure it comes upto 150F ( This will take approx 10-15 mins)
Once the mixture comes up to 150F, take it off the fire and add the mixture into a stand mixer. whisk on high speed for about 10-15 minutes until it has reached stiff peaks, it will resemble like shaving cream once it's done.
Once it has been whisked upto the desired consistency, add the butter little by little. make sure to add 1 tablespoon at a time
Melt the chocolate either in the microwave or on a double boilerIf using microwave to melt chocolate, make sure the chocolate doesnt burn, chocolate burns very quickly, so dont microwave for too long. Add the melted and cooled chocolate into the whisked meringue mixture untl well incorporated.
If the consistency is a bit loose, then keep in the refrigerator for 10-15 mins it will thicken up.Use the buttercream to decorate a chocolate cake or cupcakes