Chop the room temperature butter to roughly tablespoon sized cubes. Keep aside.
Whip the egg whites , until you have stiff peaks. If you upturn the bowl, the whipped whites should not move.
In a sauce pan with a handle , heat the sugar and water together . If you have a thermometer, let the sugar dissolve and let the sugar syrup cook till the temperature reaches between 235 F to 245 F. If you are not using a thermometer, let the sugar dissolve (you can swirl the pan or stir to help this ). Once the sugar dissolves, let the syrup cook undisturbed. Once you see bubbles, let it cook for 3-4 minutes.
Start whipping the egg whites again once the syrup has been cooking for about 3 minutes.
Pour the syrup into the egg whites (while beating) in a slow continuous stream. Pour closer to the edge of the bowl so the hot syrup doesnt touch the blades and splatter.
Once all the syrup has been added , keep whipping until the outside of the bowl is cool to touch.
Add the cubed butter, piece by piece while beating.
Once all the butter has been added, whip until you get fluffy buttercream consistency. This may take 2-5 minutes depending on your mixer.
Add a tablespoon of the pistachio paste (reserved from earlier) . Beat well.
Now add pistachio paste , one teaspoon at a time and beat well. Taste after each addition. Continue this until you have the desired flavour and consistency
Remember not to add too much at once . The buttercream will taste better but may lose its pipeable consistency. Keep the buttercream aside