Rich, creamy, custardy buttercream.
Put yolks and salt in a standing mixer and beat at high speed for 1-2 minutes.
Place sugar and water in a small saucepan. Put candy thermometer in pan. On medium heat, cook mixture to 235 degrees F.
With the standing mixer running on low speed, SLOWLY drizzle the heated sugar mixture into the centre of the yolks. If it hits the sides, it will just harden.
Remove whisk attachment and add paddle attachment.
Running the mixture at medium speed, add butter, 1-2 chunks at a time. Wait until the butter is incorporated before adding more.
When the butter is completely incorporated add salt to taste and mix in vanilla/scotch/bourbon.
If not using immediately, cool in the fridge. When you remove it from the fridge, just allow to warm at room temperature. If you beat it again, it will change its consistency. Gently smooth with a spatula before using.