Put everything in a pot and bring it to a boil. Once the sugar dissolves take it off the heat and let cool completely.
Toast the walnuts, almonds and pistachios at 180C for 5-10min or until golden and fragrant.
Let them cool down and process them until they're in fine pieces.
Cut the phyllo sheets to the size of the mould. Keep them covered by a damp kitchen cloth at all times so the phyllo doesn't dry out.
Paint butter on the base of the mould and lay 5 layers of phyllo pastry on top, also with melted butter in between each of them
Lay 1/3 of the nuts and press down with the back of a spoon.
In total you need: 5 phyllo layers on the bottom, filling, another 5 layers of phyllo, filling, 5 layes of phyllo, filling and finish with 10 layers of phyllo. Always paint with butter in between layers of phyllo.
Paint the last layer with butter.
Cut the baklava vertically and then cross those lines ina slight diagonal. You can see the detail on the video above.
Take to a preheated oven at 180C for 15-20min or until really golden on top.
Pour on the syrup AS SOON as it comes out of the oven and let it rest for at least a couple of hours so it can absorb it all in.