Step 1: Mix the egg whites and the sugar and place them over a double boiler while you whisk them together.
Step 2: The sugar will start to dissolve and the egg whites will start to turn white and fluffy. Continue whisking until the egg whites feel hot to the touch.
Step 3: Transfer the bowl to the electric mixer and whip in high speed until light and fluffy and the bottom of the bowl doesn't feel hot anymore. About 3-5 minutes.
Step 4: Start adding the softened butter by the tablespoon. Keep adding more butter as it is incorporated into the meringue.
Step 5: It might look like it is curdling but don't worry, keep mixing. When is well mixed, add the pistachio paste.