Make sure you use baking quality white chocolate to ensure proper consistency. Chips or confectioner’s bar will not work.
Place each type of chocolate in a separate bowl. Boil total amount of cream and pour specified amounts of hot cream over the corresponding chocolate. Let stand for 5 minutes then stir each bowl of chocolate with rubber spatula until smooth. Add brandy to bittersweet chocolate mixture and stir. To white chocolate, add the butter and stir until smooth.
Prepare an 8 by 8-inch pan by lining the bottom with parchment or waxed paper. Cover the parchment with plastic wrap allowing at least 5 inches of overhang.
Sprinkle bottom of pan with chopped pistachios. Pour milk chocolate mixture over top. Freeze mixture for at least 20 minutes. Pour white chocolate mixture over top of milk chocolate and smooth out with spatula until even. Freeze again for about 20 minutes or until firm. Finally, pour bittersweet chocolate over top and cover with overhanging plastic wrap. Freeze until firm. Cut into 24 even rectangles 1 by 2-inches. Sprinkle with cocoa or decorate with white chocolate curls. Let stand for approximately 30 minutes before serving.