In a food processor, combine the toasted hazelnuts, pistachios, almonds, and walnuts. Add the ground cinnamon and salt. Pulse until the nuts are finely chopped but not completely ground into a powder. Set aside.
Preheat your oven to 350°F (175°C). This will ensure that your baklava bakes evenly.
Unroll the phyllo dough and cover it with a damp kitchen towel to prevent it from drying out. Lay one sheet in your baking dish and brush it generously with melted butter. Repeat this process, layering and buttering at least 8 sheets.
Spread half of the nut mixture evenly over the buttered phyllo layers. Drizzle half of the Nutella over the nuts for an extra chocolatey layer.
Layer more phyllo sheets (about 8 more), brushing each with melted butter. Repeat the process with the remaining nut mixture and Nutella, topping with another layer of phyllo sheets.
Using a sharp knife, carefully cut the baklava into diamond shapes or squares, making sure to cut through all the layers.
Bake in the preheated oven for about 45-50 minutes, or until the top is golden brown and crisp.
While the baklava is baking, combine the water, honey, and cinnamon stick in a saucepan. Bring to a boil, then reduce to a simmer for about 10-15 minutes. Remove the cinnamon stick and let the syrup cool.
Once baked, remove the baklava from the oven and immediately pour the cooled syrup evenly over the hot baklava. Allow it to absorb the syrup for at least 30 minutes before serving.