In a small saucepan, combine sugar and water. Bring it to a boil over medium heat, stirring it constantly until sugar is completely dissolved. Then continue to cook until the syrup reaches 250°F (120°C), about 5 minutes.
Simultaneously, start beating egg yolks in a mixing bowl with whisk attachment until it's doubled in size and becomes pale in color, about 2 minutes.
While the mixer is running on low speed, slowly pour the hot syrup into the egg yolks. Pour the syrup into the side of the bowl, instead of the whisk, so that it doesn't splatter.
Increase the speed to medium high, and beat the mixture until it cools down to 104°F (40°C). The mixture will become smooth and white.
Stir in butter one tablespoon at a time.
Then add pistachio paste and food coloring, if desired. Continue to beat the cream until nice and smooth.