Preheat the oven: Begin by preheating your oven to 350°F (175°C). While the oven heats up, grease a 9x13 inch baking dish with 2 tablespoons of the melted butter.
Prepare the nut filling: In a medium bowl, combine 1 cup of chopped walnuts, 1 cup of chopped pistachios, 1 teaspoon of ground cinnamon, and 2 tablespoons of sugar. Mix well to incorporate the ingredients evenly.
Layer the phyllo dough: Unroll the package of phyllo dough and cover it with a damp towel to prevent it from drying out. Take one sheet and brush it with 2 tablespoons of melted butter. Place it in the greased baking dish. Repeat this step for a total of 8 sheets, brushing each layer with butter.
Add the nut mixture: Spread 1 cup of the nut mixture over the phyllo layers evenly. Repeat the process of layering 8 more sheets of phyllo, brushing each with butter, followed by another layer of the nut mixture. Continue this process until you have used all of the filling, finishing with 8 layers of phyllo on top.
Cut the baklava: Using a sharp knife, carefully cut the assembled baklava into diamond-shaped pieces. This step is essential because it allows the syrup to penetrate the layers more effectively.
Bake the baklava: Place the baking dish in the preheated oven and bake for 45-50 minutes, or until it turns golden brown and crisp on top.
Prepare the syrup: While the baklava is baking, combine 1 cup of sugar, 1 cup of water, 1 cup of honey, and 1 teaspoon of vanilla extract in a saucepan. Bring the mixture to a boil over medium heat and let it simmer for 10 minutes until it thickens slightly.
Soak the baklava: Once the baklava is finished baking, remove it from the oven and immediately pour the hot syrup over the top, ensuring it seeps into the cuts you made.
Cool and serve: Allow the baklava to cool for a few hours at room temperature before serving, which will let the syrup soak in and flavors meld beautifully.