2: Divide the blackcurrant crémeux evenly between the glasses.
3: Leave to cool to room temperature, then refrigerate for half an hour.
4: Once the crémeux is cold and set, fill up the glasses with chestnut mousse.
5: Just before serving, stick a sablé biscuit into the top. Serve without delay.
Remarks
You can use a different fruit instead of blackcurrants in the crémeux, but do try to keep to something with a hint of acidity (raspberries, currants, etc.), to contrast with the chestnut flavour.