Top and tail the gooseberries and rub off any fibres. Put them into a pan and add half the sugar. Cook over a gentle heat until bursting. When soft and fragrant, set aside to cool.
Warm the milk in another pan. In a bowl, whisk the egg with the cornflour, the remaining sugar and vanilla until combined. Pour the warm milk over the egg and cornflour, whisking all the time. Then return to the pan and whisk until thickened. Allow just to bubble before taking off the boil whisking all the time. Cool.
Whip the cream to soft peaks. Tip the gooseberries into a blender or food processor and pulse to puree. Fold the puree and cooled custard together and then fold in the cream. Chill before serving.
Alternatively use 200ml (7fl oz) ready-made custard, and combine with gooseberry puree and whipped cream.
Heap into glasses, chill and serve.
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