Preheat oven to 350 degrees F. Butter a 15x10x1-inch baking pan.
PREPARE THE BAKLAVA:
Layer eight sheets of phyllo dough (one at a time) in the prepared pan, brushing each with butter before adding the next sheet. In a bowl, combine the nuts, chocolate chips, sugar, cinnamon and lemon zest. Sprinkle 2 cups over the top layer of phyllo.
Add four more sheets of phyllo dough (one at a time), brushing each with butter before adding the next. Top with 2 more cups of the nut mixture. Layer and brush four more sheets of dough with butter; top with the remaining nut mixture. Top with the remaining dough, brushing each sheet with butter. Drizzle any remaining butter over top.
Using a sharp knife, cut the baklava into 1½-inch diamonds. Bake for 40 to 50 minutes or until golden brown.
PREPARE THE SYRUP:
While the baklava is baking, combine the syrup ingredients in a saucepan; bring to a boil over medium heat, stirring occasionally. Reduce the heat; simmer, uncovered, for 20 minutes. Pour the syrup over the warm baklava. Cool completely in pan on a wire rack before you remove the pieces of baklava from the pan.
Tip for working with phyllo dough- you don't want it to dry out. If you get some paper towels wet and wring them dry as much as possible, you can lay them on top of the phyllo to keep it damp. You don't want to get the phyllo 'wet,' but if you can keep it from drying out- that will help prevent tearing as you use it.