For the crackle, place the sugar in a saucepan over a medium heat and cook for five minutes, until it caramelises.
Remove from the heat, add the lemon juice and stir through hazelnuts.
Drizzle out onto non-stick baking paper and leave to set.
Meanwhile, for the trifle, break up four of the sponge fingers and place in the bottom of a tall ice cream glass.
Add the scoops of ice cream.
Place the cream in a bowl and add the vanilla seeds. Lightly whip the cream and vanilla, then add the mascarpone and stir through.
Place the mascarpone cream into the ice cream glass and drizzle with melted chocolate.
Top with the two remaining fingers and add some of the hazelnut crackle.
Garnish with a sprig of mint and serve.