Preheat the oven to 350°F. Brush melted butter on a 10"x15" baking pan. If using 9"x13" pan, you might have to cut the phyllo pastry a little bit to fit the pan.
Place one phyllo sheet in the baking pan and brush with melted butter. Place another sheet and brush with butter. Repeat until you have 6 layers of phyllo. Spread ⅓ of the Nutella on top and sprinkle with ⅓ of the nut mixture.
Repeat the layering process: 6 sheets of phyllo (each brushed with butter) + Nutella + Nuts.
So in the end this is how the baklava should be:
1. 6 sheets of phyllo (each brushed with butter) + Nutella + Nuts
2. 6 sheets of phyllo (each brushed with butter) + Nutella + Nuts
3. 6 sheets of phyllo (each brushed with butter) + Nutella + Nuts
4. 6 sheets of phyllo (each brushed with butter)
Brush the top most layer of phyllo with butter. Using a sharp knife cut the phyllo into squares or diamonds. Bake for 35 minutes or until the phyllo pastry is golden brown on top and crispy around the edges.