For the raspberry-blackberry jam, wash the blackberries and raspberries. Put them in a large pot and add the preserving sugar.
Heat the whole thing until a little juice has formed. Then puree the fruit and boil the jam for 5 minutes. Finally, add a dash of whiskey.
To check if the jam is ready, be sure to do the jelly test. To keep the raspberry-blackberry jam as long as possible, it is best to wash the jam jars as hot as possible before filling.