Preheat the Oven: Preheat your oven to 350°F (175°C).

Prepare the Filling: In a bowl, mix the chopped pistachios with cinnamon. Set aside.

Assemble the Baklava: Brush a 9×13 inch baking dish with melted butter. Layer 8 sheets of phyllo dough in the dish, brushing each sheet with melted butter before adding the next. Evenly spread about 1/3 of the pistachio mixture over the phyllo. Layer 5 more sheets of phyllo, again brushing each with butter. Repeat this process twice more, finishing with a top layer of 10 buttered phyllo sheets.

Cut the Baklava: Using a sharp knife, cut the baklava into diamond shapes or squares.

Bake: Bake in the preheated oven for about 50 minutes or until golden brown and crisp.

Prepare the Honey Syrup: While the baklava is baking, combine honey, sugar, lemon juice, and water in a saucepan over medium heat. Bring the mixture to a boil, reduce the heat, and let it simmer for 10 minutes. Remove from heat and stir in vanilla extract.

Pour the Syrup: Once the baklava is done baking, immediately pour the hot syrup evenly over the top. Let the baklava cool and absorb the syrup for at least 4 hours, preferably overnight.