Preheat the oven to 180°C.
Grease and line a 20x30cm rectangular cake tin with baking parchment.
Beat butter and sugar until pale and fluffy, then gradually add eggs and vanilla extract.
Sift flour and cocoa powder into a bowl and fold into the butter mixture.
Spoon the batter into the tin and smooth the surface.
Bake for 30 minutes until a skewer comes out clean.
Cool the cake in the tin for 10 minutes then transfer to a wire rack.
Rub butter into flour, ginger, and bicarbonate of soda until it reaches a breadcrumb texture.
Mix in sugar, syrup, and egg to form a soft dough, then chill for 30 minutes.
Preheat the oven to 180°C.
Roll out the dough on a floured surface and cut into tombstone shapes.
Bake biscuits for 10-12 minutes and cool on a wire rack before decorating with icing.
Beat butter, icing sugar, cocoa, and milk until the buttercream is light and fluffy.
Pulse Oreo or bourbon biscuits into fine crumbs using a food processor.
Spread buttercream over the cooled cake.
Stick halved chocolate sticks around the sides of the cake to create a fence.
Sprinkle biscuit crumbs on top and add the tombstone biscuits and sweets.