Grease and line a 20cm (8") square cake tin and set aside (see my notes).
Place Gingernut biscuits in a food processor, then process into crumbs then set aside.
In a large microwave safe bowl, melt butter (70g) then add condensed milk (80g) and ground ginger (1 tsp - if using) and mix. Then add golden syrup (1 tbsp) and mix to incorporate.
Add biscuits crumbs and mix together well. Press biscuit mixture into the base of your tin, and place the tin in the fridge to start setting, while you melt the chocolate for the topping.
Place dark chocolate (200g) into a microwave safe bowl, along with the remaining butter (50g). Microwave in 30 second bursts (stirring between bursts with a metal spoon) until chocolate is melted and smooth.
Pour chocolate in an even layer over the top of the biscuit layer, smooth the top and refrigerate for 2 hours or until set.