Soak the gelatine leaves in a bowl of chilled water for 5 minutes, blitz the raspberries to a fine purée. Pass through a sieve into a clean bowl, discarding the pips, then stir in the raspberry liqueur.
Now mix the cornflour with a little of the milk in a pan. Add the remaining milk, along with the sugar and ground almonds. Bring to the boil, then reduce the heat and simmer gently for 3–5 minutes, to thicken.
Squeeze out any excess water from the gelatine leaves, then add them to the pan, stirring continuously until dissolved. Add the almond extract. Stir in 300ml of the double cream and all the raspberry purée, combine thoroughly, then pour into the mould. Leave to set for at least six hours, preferably overnight.
To serve, lightly whip the remaining 150ml of cream to soft peaks, then spoon into a medium piping bag fitted with a medium star nozzle.
Remove the mould from the fridge and quickly dip it into a bowl of hot water to loosen the blancmange. Invert the mould onto a serving plate to turn out the blancmange.
Finally, pipe the cream around the base of the blancmange, then decorate with crystallised rose petals.