Preheat the oven to 180°C (356°F).
Cream the butter and sugar together.
Add the egg and mix well.
Sift cake flour, baking powder, and salt together, then add to the mixture.
Add finely chopped pistachios and white chocolate, and mix well.
Shape the dough into balls and place them on a baking sheet.
Bake in the oven at 180°C (356°F) for 12–15 minutes.
Let the cookies cool to finish.