For the cookies: Place the flour and salt in a mixing bowl. Add the butter and rub with your fingers until the mixture resembles breadcrumbs. Add the sugar and work it until a dough is formed, using a sprinkle of cold water if need be. Mix in the cranberries and pistachios, roll into a cylinder and divide into 3 pieces. Cover each piece separately with plastic wrap and place in the freezer until chilled.
For the sauce: Scoop out the pulp and seeds from the passionfruit and add to a food processor with the juice and sugar. Whizz until blended, about a minute. Pass through a muslin cloth (or cheesecloth) into a jug and chill until needed.
To bake the cookies: Preheat the oven to 180 degrees C (390 degrees F). Line a baking sheet with parchment paper.
Remove one dough portion from the freezer and allow to thaw slightly. (You can keep the other dough portions in the freezer for another occasion.) Slice into 12 rounds and place on the prepared baking sheet. Bake until slightly golden, 12 to 15 minutes. Let cool slightly before transferring the cookies to a wire rack to cool completely.
For serving: Put 2 scoops of vanilla ice cream into each serving bowl, top with a cookie and drizzle with passionfruit sauce.