Put the frozen redcurrants and blackcurrants in a bowl with half of the sugar and defrost in the microwave oven for 5 minutes on MEDIUM.
Heat the milk in a cup on HIGH for 21/2 minutes.
Whisk the eggs with the remaining sugar in a bowl.
Add the cornstarch (cornflour).
Pour in the hot milk in a thin stream, whisking vigorously.
Stir in the sour cream (creme fraiche) and microwave, uncovered, for 5 minutes on HIGH, whisking after every minute.
Add the fruit and the raspberry liqueur.
Microwave, uncovered, on HIGH for 8 minutes, stirring twice during cooking time.
Stir the mousse again, then refrigerate, stirring from time to time to prevent a skin from forming.