Heat oven to 325°F.
Layer 10 sheets of phyllo pastry into the bottom of the pan, brushing a thin layer of melted butter between each layer and keeping the unused phyllo dough covered with a damp cloth to prevent it from drying out.
Sprinkle approximately ⅕ of the chopped walnuts and cinnamon (roughly ¾ cup) in a thin, even layer over the first 10 pastry sheets.
Repeat 4 more times but with 5 sheets of phyllo dough instead of 10, brushing each with melted butter before adding the next sheet. Sprinkle ¾ cup of the chopped walnut mixture after every 5 layers of buttered phyllo dough.
Finish the baklava with a final, top layer of phyllo dough brushed with butter between the layers using the remaining 6-10 sheets of dough. Brush the top sheet of phyllo dough with butter as well.
Carefully slice the baklava into 1 ½-inch wide strips lengthwise, then slice diagonally to create diamond-shaped baklava. Or just slice squares if you prefer.
Bake in the 325°F oven for 1 hour and 15 minutes, until the baklava is golden brown. Remove from the oven and immediately drizzle all of the cooled syrup over the baklava.
Let the baklava cool completely at room temperature for 4-6 hours without being covered so the baked pastry can soak up all of the syrup.