Measure 100ml (31⁄2fl oz) boiling water into a heatproof jug and add the jelly cubes, stirring until dissolved. Add the elderflower cordial and enough cold water to make the liquid up to 500ml (17 1⁄2fl oz). Divide the liquid jelly between 6 sterilised jam jars and chill for at least 3 hours or overnight to set.
To complete the jelly jars (up to 2 hours in advance), divide the berries and cake cubes between the jars. Screw on the lids and chill until needed. To serve, remove the lids and top each jar with a generous spoonful of crème fraîche.