Defrost the phyllo dough according to the package instructions.
Preheat the oven to 325°F.
Melt the butter in the microwave and remove the foamy white layer on top to clarify the butter.
Pulse the walnuts, cinnamon, and sugar together in a food processor and set aside.
Assemble the baklava: first brush a layer of butter over the pan and place 10 layers of dough in the bottom of the pan. Brush the last dough sheet with butter and spread a layer of the nut mixture over top. Repeat this process two more times and then place the remaining dough layers (10) on top, buttering between every top dough sheet.
Cut the baklava using a sharp knife by first cutting parallel lines across the pan, and then cut diagonally across those lines.
Bake in the oven for two hours until the top layers are golden and crispy.
While the baklava is baking, make the syrup by boiling the water and sugar for 5 minutes. Reduce to a simmer and add in the honey and vanilla until dissolved into the syrup. Allow to cool while the baklava finishes baking.
As soon as you pull the baklava out of the oven, pour the syrup over it and allow it to cool before serving.