Split the kumquats in half lengthwise. Use a spoon to scoop the flesh out into a bowl, leaving the skin intact. Set the skins on a baking sheet. Press the kumquat flesh through a sieve over a bowl to squeeze out the juice; discard the pith and seeds. Measure out 3 tablespoons (45g) of kumquat juice and set aside.
Bring the heavy cream and sugar to a boil in a non-reactive pot (see headnote). Boil for 3 minutes. Remove from heat and stir in kumquat juice. Let stand in the pot to cool for 10 minutes. Stir the mixture, then pour into a jug or cup measure with a spout. Fill the kumquat skins almost to the top with cream. Pour any remaining cream into a ramekin to enjoy on the side. Cover and chill for at least 1 hour, until set. Sprinkle with chopped pistachios.