Place the yolks in the bowl of a stand mixer and fit the mixer with the whisk attachment. Set aside.
Add the sugar to a small 2-quart saucepan and carefully pour the water around the edges. Don't stir it! Turn the heat on to medium and allow the mixture to come to a boil WITHOUT stirring!
As soon as the sugar syrup begins to boil, clip on a candy thermometer. When the mixture reaches around 220° F, begin whisking the egg yolks on medium.
When the sugar syrup reaches 239° F, immediately remove from the heat. With the mixer running at high speed, slowly and carefully pour hot sugar syrup into egg yolks, trying to reach the spot between the whip and the sides of the bowl, or right about the edge of where the yolks meet the sides.
Continue to whip until the mixture is light, fluffy, and cool to the touch.
Gradually beat butter into the yolk mixture, a few tablespoons at a time, until all the butter is incorporated. Mixture may look curdled during this stage. Just keep mixing and it will smooth out.
Flavor as desired. Use immediately or refrigerate until ready to use. Store in air tight container up to two weeks in the refrigerator or freeze for up to three months. Recipe can be doubled or even tripled.