Blend the raspberries and icing sugar in a food processor until smooth.
Sieve the sauce into a bowl and adjust sweetness with extra sugar if needed.
Whisk the double cream in a bowl until soft peaks form.
Gently fold the crème fraîche and vanilla seeds into the cream.
Heat caster sugar in a pan until it melts into a golden-brown caramel.
Drizzle the caramel over a clean knife steel to create thin strands.
Pinch the cooled strands together and set aside.
Place a spoonful of sauce and one shortbread onto each of four plates.
Layer the shortbread with vanilla cream and repeat twice more to stack.
Dust the top of each stack with icing sugar.
Use a heated metal skewer to mark a caramelised cross on the sugared tops.
Garnish with the spun sugar strands and serve.