Preheat Oven to 325˚F.
Pulse walnuts 10 times in a food process or until coarsely ground/ finely chopped. In a medium bowl, stir together: 4 cups finely chopped walnuts and 1 tsp cinnamon.
Place 10 phyllo sheets into baking pan one at a time, brushing each sheet with butter once it’s in the pan before adding the next (i.e. place phyllo sheet into pan, brush the top with butter, place next phyllo sheet in pan, butter the top, etc. etc.). Keep remaining phyllo covered with a damp towel at all times. Spread about 1/5 of nut mixture (about 3/4 cup) over phyllo dough.
Add 5 buttered sheets of phyllo, then another layer of nuts. Repeat x 4. Finish off with 10 layers of buttered phyllo sheets. Brush the very top with butter.
Cut pastry into 1 1/2″ wide strips, then cut diagonally to form diamond shapes. Bake at 325˚F for 1 hour and 15 min or until tops are golden brown.
Remove from oven and immediately spoon cooled syrup evenly over the hot baklava (you'll hear it sizzle). This will ensure that it stays crisp rather than soggy. Let baklava cool completely, uncovered and at room temp. For best results, let baklava sit 4-6 hours or overnight at room temperature for the syrup to penetrate and soften the layers (if you can keep your hands to yourself!) Garnish baklava with finely chopped nuts or drizzle with melted chocolate. Store at room temp, covered with a tea towel for 1 to 2 weeks