First, decorate the glasses.
Pour the sprinkles into a small bowl.
Snap the chocolate into a small heatproof bowl and either melt in the microwave in 30-second blasts, stirring between each go, or over a pan of simmering water, shallow enough that the water doesn’t touch the bowl.
When melted, brush chocolate around the top of each glass in a 1/2 cm-thick band on the inside and outside (or you can did the rim of the glass in but make sure to let excess chocolate drip off).
Dip each glass into the sprinkles and leave to set.
Toast the hazelnuts in a dry frying pan over a medium heat, tossing regularly, until golden.
Blitz the cookies in a food processor, or seal in a food bag and bash with a rolling pin, until you have rough crumbs.
Stir the hazelnuts into the cookie crumbs and set aside.
If using unsweetened chestnut purée, blend in a food processor to get rid of any graininess.
Beat the mascarpone and sugar in a medium bowl until smooth.
Fold the chestnut purée into the mascarpone until well mixed. (If using sweetened chestnut purée simply beat with the mascarpone and omit the sugar.)
To assemble, spoon a tablespoon of crumb mixture into each glass.
Top with 2 tablespoons of the creamy mixture, being careful not to mess up the sides or top.
Add second layers of biscuit and creamy filling and finish with biscuit.
You can serve straight away, or for a firmer finish, cover with cling film and chill for 1 hour.
When ready to serve, softly whip the cream in a medium bowl.
Dollop a spoonful onto each sundae, sprinkle with edible pearls and sit a cherry proudly on top, stalk upright.
For ultimate cute factor, decorate with homemade flags.