Cut the tofu into 3 or 4 slabs. Place the slabs on a paper towel-lined plate or cutting board and cover with more paper towels. Place another plate or cutting board on top and then place something heavy (like a saucepan filled with water) on top of that. Leave the tofu to drain for 30 minutes.
Place the drained tofu into a blender with the golden syrup, chocolate, vanilla and salt. Add 2 tbsp of water and blend again. The mixture should have the consistency of a thick-yet-spreadable ganache - blend in the remaining water if the mixture seems too thick.
Line a small loaf tin or small round cake tin with cling film (plastic wrap). Spoon a few tablespoons of the tofu ganache into the base of the tin and spread it out into an even layer. Add a layer of digestive biscuits (I could fit 2 into the base), then layer with the ganache. Continue layering biscuits and ganache until you've used up all the ganache. (You may need to break biscuits in half to get them to fit into the pan!). Chill the cake for at least 4 hours, preferably overnight.
Unmould the cake and remove the cling film. If desired, use a hot palette knife (dip it into boiling water for a few seconds then wipe dry) to smooth the outside of the cake. Drizzle with the 20 g of melted chocolate and quickly sprinkle with the pistachios then leave to set. Finally, slice & serve! Store any leftovers in the fridge.