While the syrup cools, make the pastry.
Put the nuts, sugar, and cinnamon in a food processor and process until fairly fine.
Lay out your phyllo and cut it so that it will fit in your pan. We made a large batch on a baking sheet and used two sheets to cover the bottom, but cut them so that they fit width-wise. If using a half sheet tray or another dish for baking, adjust your phyllo as needed. Set aside enough sheets of dough to have 8 layers of dough on top.
Brush the bottom of the pan with clarified butter and lay down one layer of phyllo. Use a brush to butter it, and lay on another. Do this until you have 8 layers of phyllo dough with butter between each one. This is the base of your pastry. Scatter a third of the nut mixture over the surface of the dough. Spread it evenly over the whole surface.
Add another layer of phyllo, butter it, put on another layer, and butter it. Now add half the remaining nuts.
Add two more layers of dough, each with butter between, and add the rest of the nut filling.
Put on the top of the pastry by layering on the reserved sheets of dough, each with butter between, until the top is 8 layers thick. Pour on any unused butter.
Put the pastry in the refrigerator for 45 minutes for the butter to solidify a little. While the pastry firms, preheat your oven to 350°F (177°C)
Remove the pastry from the fridge and cut your desired shape pieces using the tip of a sharp knife.
Bake your baklava for 30–50 minutes. (Use a timer to keep track of it!) When the baklava is golden brown and gorgeous looking, it is done. Remove the baklava from the oven and strain the syrup over the top of it. Allow the baklava to cool and absorb the syrup for at least 4 hours, but overnight is better.
Plate and serve your baklava to your adoring family and friends.