Toast the pistachios (either in the oven or stovetop will work) and chop them roughly.
Combine double cream with 1 cup chopped pistachios; mix then set aside in the refrigerator for at least 30 minutes.
Take the cream with pistachios from the refrigerator and strain the pistachios. Set it aside.
Add vanilla extract into the cream and whip it to combine.
In a separate medium-sized bowl, beat the egg whites while slowly adding the sugar -- continue beating until the egg whites become stiff and glossy.
Gently fold 1/3 of the egg whites into the whipped cream; repeat until all the egg whites have been combined with the cream.
Add the strained pistachios and mix gently.
Spoon into your molds or tin. Cover the tins with plastic wrap.
Place tins or molds in the freezer for at least 4 hours.
Remove your pistachio semifreddo from the tin and sprinkle with the remaining chopped pistachios.
Serve.