Preheat the oven to 350°. In a food processor, finely chop the walnuts with the cinnamon, cardamom and 1/2 cup of the sugar. Transfer to a bowl and stir in the chocolate chips.
Brush an 11-by-17-inch metal baking pan with some of the melted butter. Lay a sheet of phyllo in the pan; trim the edges to fit and brush with butter. Repeat with 7 more phyllo sheets, brushing butter between each layer. Spread 2 1/2 cups of the walnut filling evenly over the phyllo. Top with another 8 sheets of butter-brushed phyllo and the remaining 2 1/2 cups of walnut filling. Cover with 8 more buttered phyllo sheets, buttering the top well. Refrigerate the baklava for at least 30 minutes and up to 1 day.
With a small, sharp knife, cut the baklava into diamonds, making sure to cut all the way through to the bottom. Bake the baklava for 40 minutes, or until nicely browned. Remove from the oven and let cool completely.
In a small saucepan, combine the remaining 1 3/4 cups sugar with the water and bring to a simmer over moderate heat. Stir until the sugar is dissolved and the syrup is clear. Pour the hot syrup evenly over the baklava and let cool completely before serving.
The baklava can stand at room temperature for up to 1 day.