Boil 600ml double cream and 150g sugar for 3 minutes over low heat while stirring.
Whisk in lemon juice and zest once cooled, pour into bowls, and chill for 3 hours.
Preheat oven to 65°C and line a tray with a silicone mat or parchment.
Whisk egg whites to stiff peaks, then gradually add sugar until glossy.
Spread meringue thinly on the tray and bake for 3 to 4 hours until dry.
Break the dried meringue into shards and set aside to cool.
Increase oven temperature to 140°C and grease a 30x20cm cake tin with butter.
Melt sugar, butter, golden syrup, and treacle in a small saucepan over low heat.
Combine dry ingredients with eggs, milk, and the melted syrup mixture until smooth.
Pour mixture into the tin and bake for 75 minutes until a skewer comes out clean.
Top possets with crumbled gingerbread, lemon balm, meringue shards, and optional sorbet.