Line a 30cm circular baking tray with cling film.
Heat butter, sugar and syrup in a saucepan until combined.
Remove from heat and stir in cocoa powder and puffed rice cereal.
Press the warm crispy mixture into the tray and chill until set.
Preheat the oven to 180°C.
Beat sugars and butter in a bowl until light and fluffy.
Stir in the egg and vanilla extract until combined.
Sift in flour, salt, baking powder, and ground ginger before mixing into a dough.
Mix in chocolate pieces and press dough into a lined 30cm tray 1cm deep.
Bake for 25 to 30 minutes until golden but soft in the middle.
Cool the cookie on a wire rack completely.
Whisk double cream and icing sugar until lightly whipped.
Stir in ginger syrup and chopped ginger.
Spread ginger cream over the crispy base and scatter grapefruit zest.
Place the cookie on top to assemble.