Cut the plums in half vertically and remove the stone from the centre.
In a baking dish large enough to fit the halved plums in a tight single layer, approximately 230mm x 230mm in size, place the sugar, cinnamon quill, vanilla, and lemon zest and spread into an even layer.
Arrange the plum halves, cut side down, on top of the sugar.
Roast in the preheated oven for 30 minutes, or until the plums are tender but still hold together.
While the plums are still warm, carefully remove the skins and return them to the baking dish.
Allow to cool at room temperature for 1 hour before transferring the dish into the refrigerator to chill for approximately 3 hours.
Spun Sugar Balls
Fill a bowl, large enough to submerge the base of a large saucepan into, with cold water and ice.
Place the sugar and water into a large saucepan over an induction cooktop on setting 7 and bring to the boil.
Continue to boil until the sugar is completely melted and reaches a light caramel colour.
Remove from the heat and immediately dip the base of the saucepan into the ice water for a few seconds.
Set aside and allow the caramel to cool slightly.
Dip two forks into the caramel and flick them quickly over a sheet of baking paper to create thin strands of sugar. Continue this process to create more sugar strands.
Carefully pick up the spun sugar and curl it into a ball.
Repeat this process another 3 times to create a total of 4 spun sugar balls.
Reserve the spun sugar balls in the freezer until you are ready to garnish the trifles.
Assembly
In a bowl, mix the dollop cream together with the sieved icing sugar. If the cream is still liquid, gently whisk by hand until you achieve a soft and supple texture.
Reserve 4 of the best looking roasted plum halves to use on top of the trifles, then cut the remaining roasted plums into small pieces.
In 4 individual trifle glasses, layer the plum pieces with their syrup, shortbread, cream and shaved white chocolate.
Finish the final layer with cream, then place a roasted plum half on top, flat side down.
Prior to serving, garnish each trifle with a spun sugar ball.
