Thaw the phyllo dough overnight in the fridge and then let it sit on the counter for about half an hour. Put it aside.
Prepare the syrup. Put the syrup ingredients in a pot. Heat until it boils. Let it simmer over medium low heat for 15 minutes and cool completely.
Trim the phyllo sheets to fit your baking pan. We used 25 sheets in this recipe and trim them all together. Cover them with a damp cloth so that the sheets won’t dry out when you are working. Each time you place a phyllo sheet into the pan, make sure you cover the rest. They can easily dry out otherwise.
Preheat the oven to 350F.
Butter the bottom and the sides of a 9x13 baking pan.
Place one phyllo sheet in the pan and brush it with melted butter. Place the second phyllo over it and butter it too. Repeat this until you have 5 sheets in total.
Spread ½ cup walnuts over the 5th sheet evenly.
Place 5 phyllo sheets on the walnuts, brushing each with melted butter. Sprinkle walnuts on the 5th.
Repeat with another 5 phyllo sheets and walnuts on the 5th.
Repeat with another 5 phyllo sheets and walnuts on the 5th.
In total, you will have 25 phyllo sheets and 4 layers of walnuts (between each 5 phyllo sheets).
Make sure you butter the top as well.
Using a sharp knife, cut the raw baklava into vertical strips and then cut diagonally. Make sure you cut it all the way to the bottom of the pan. Bake it at 350F for 30 minutes or until tops are golden.
Remove it from oven and using a ladle, pour cooled syrup over hot baklava. Let it sit uncovered for 4-5 hours so that the syrup spreads well into the layers.
Garnish with ground pistachio and serve.