To make the brittle, put the granulated sugar and almonds in a saucepan over a low heat. While the sugar melts, lightly oil a baking sheet. When the sugar has melted and turned golden, pour the mixture on to the baking sheet and leave to cool.
To make the syllabub, pour the coconut cream and double cream into a large bowl. Add the crushed cardamom seeds and caster sugar, then lightly whip until just holding soft peaks.
Spoon the syllabub into 4 glasses and chill in the refrigerator. Meanwhile, lightly crack the brittle into irregular shards. When ready to serve, top the syllabub with some of the brittle and serve the remainder separately on the side.