Preheat oven to 350°F. Unwrap the phyllo dough and wrap it with a damp cloth to prevent it from drying out. Set aside.
16 ounces phyllo dough
In a medium-sized bowl, combine the walnuts, cinnamon, and salt. Set aside.
1 pound finely chopped walnuts, 2 teaspoons ground cinnamon, ½ teaspoon kosher salt
Using a pastry brush, coat the bottom and sides of a deep 9x13-inch baking dish with butter.
2⅓ cups unsalted butter
Place 2 sheets of phyllo dough into the bottom of the pan. Brush with butter. Place 2 sheets of phyllo dough on top and brush with butter. Repeat these steps until you have 8 sheets of phyllo dough in a stack. Brush with butter before sprinkling about 2-3 tablespoons of the walnut mixture over the surface of the dough. The entire surface of the phyllo dough will not be covered with the mixture. That is fine, as with the amount of layers that there will be, there will be plenty of walnut mixture in every bite.
Place 2 sheets of phyllo dough on top of the walnut mixture. Brush with butter. Sprinkle 2-3 tablespoons of the walnut mixture over the surface. Continue this same process until you have about 6 sheets of phyllo dough remaining and no walnut mixture. A few of my sheets got sticky from the damp cloth (I only had 5 sheets remaining). Place 2 sheets of dough on top and brush with butter. Repeat until you have used all of the phyllo dough. Brush the top with butter. At this point, you may have extra butter, which is fine. If at any point you run out of butter, simply melt another tablespoon and continue.
Score the top 6 layers of dough in the final shape that you want to cut them (see notes), but do not cut completely through to the bottom of the dough. The dough at the bottom of the pan should remain uncut so as to prevent the syrup from soaking through and making it soggy.
Place the baklava into the preheated oven to bake for 45 minutes.
While the baklava is baking, make the syrup. Place all of the syrup ingredients into a large saucepan set over medium heat. Whisk constantly for the first minute while everything is melting together. Once the mixture has melted together, leave to reach a boil. Once it begins to boil, reduce to medium-low heat to simmer for about 10 minutes, or until it has thickened significantly. The mixture should coat the back of a wooden spoon. Be sure to whisk intermittently while it simmers as it could scorch or boil over. Once thickened, remove from the heat for a moment or two while the baklava finishes baking.
1 cup granulated sugar, 1 cup water, 10 tablespoons honey, 1 teaspoon pure vanilla extract, 1 teaspoon ground cinnamon, ¼ teaspoon allspice
Once the baklava has finished baking, remove from the oven and place onto a cooling rack. Pour the hot syrup over the entire surface of the baklava immediately. Let set for about 30 minutes to fully soak in and cool down.
Once cooled sufficiently, use a sharp knife to finish cutting all the way down through the baklava. Serve immediately, or allow the syrup to continue to soak for 8-24 hours.