· Preheat oven to 180°C.

· Grease the base and sides of a 42cm x 30cm low-sided roasting tray with baking spray or additional melted butter.

· To make the base, place butter and sugar into the bowl of an electric mixer fitted with a paddle attachment. Beat until pale and fluffy. Add lemon zest and combine.

· Sift flour, baking powder and ginger into mixture and mix until well combined and mixture holds together. Weigh mixture and set aside 400g.

· Place remaining 1.15kg of dough firmly and evenly into tray. Use a metal scraper of knife to run along top edges to even out. Bake for 25 minutes or until golden brown. Remove from oven and place on a rack to cool.

· To make the topping, place butter, golden syrup, condensed milk and ground ginger in a medium-sized saucepan over low heat. Stir continuously until butter is melted and amalgamated into condensed milk and mixture is caramel coloured.

· Pour onto cooked base and spread over evenly.

· Take the reserved 400g base mixture and squeeze in the palm of your hand to bind. Break off chunks and scatter over top of ginger caramel. Sprinkle walnuts and crystallised ginger on top.

· Bake for 20 minutes or until ginger caramel is dark golden brown and set. Transfer to a cake rack to cool completely then remove from tin. Neaten edges with a sharp knife. (Reserve trimmings to make truffles or unbaked fudge cake.)

· Place chocolate in a small bowl over a saucepan half-filled with water. Set over medium heat and stir until chocolate is melted. Use a piping bag with a small nozzle or a table fork to create lines of chocolate in a zigzag fashion over surface of slice. Leave at room temperature to set.

· The trimmed slice will be about 40cm x 28cm. To avoid wastage, divide the longest side into 5 and the shortest side into 8 to get slices of about 8cm x 3.5cm. Store in an airtight container with paper liners in between each layer for up to 1 week or freeze for up to 3 months.