Chop the white chocolate and place in a microwave-safe bowl.
Warm up the white chocolate in the microwave, in 30 second intervals, until melted evenly.
Continue to microwave the chocolate, mixing with a hand whisker between each interval. This process can take a while, so be patient and keep whisking. Eventually, the chocolate will start to caramelize until it is a nice golden color. Let it cool down completely.
In the meantime, beat the heavy cream in the mixer to stiff peaks. Set aside about 100 grams for the decoration.
Melt the butter and whisk it into the cooled chocolate.
Gently fold in the the whipped cream in the mixture.
Spread this cream on the surface of the pie and smoothen it over with a spatula. Refrigerate for at least 8 hours.
Remove the pie from the pan and decorate with remaining chopped pistachio and whipped cream.